JUBA, South Sudan - Uganda's president on Monday warned South Sudan's rebel leader against rejecting the government's offer of a cease-fire, saying regional leaders would unite to "defeat" the former vice president who is accused of mounting a failed coup in the world's newest country.

Ugandan President Yoweri Museveni told reporters in Juba, the South Sudan capital, that a regional bloc known as IGAD had given Riek Machar "four days to respond" to the cease-fire offer.

"If he doesn't we shall have to go for him, all of us," he said, referring to IGAD.

A meeting of East African leaders last week said it "welcomed the commitment" by South Sudan's government to cease hostilities against rebels and urged both sides to start peace talks by Tuesday. Machar instead called for a negotiated cease-fire that includes a way to monitor compliance.

Violence since mid-December in South Sudan has displaced up to 180,000 people, the United Nations said Monday.

Uganda's influence is strong in South Sudan, where special forces from the neighboring country have been deployed at the request of South Sudan's President Salva Kiir, raising questions about the impartiality of Uganda as a possible mediator in a conflict that many fear could lead to civil war in the world's newest country.

But France's ambassador to the United Nations said South Sudan's government has the right under international law to seek help from neighboring countries to defend itself.

"There is a government in South Sudan, which has the right to ask for another country to support its military efforts," said Gerard Araud, who is the UN Security Council president. He added that the Security Council "does not have a say" in whether South Sudan seeks such help.

Museveni and Kiir are strong allies. The Ugandan leader is believed to be concerned about the security implications for Uganda of a violent takeover of South Sudan's government.

For years the brutal warlord Joseph Kony, who once operated in the expansive jungle that now falls within South Sudan's territory, was a source of tension between Uganda and Sudan. Sudan's government faced persistent allegations of supporting Kony's rebellion against Uganda's government. Kony was forced to flee, and is thought to have fled to Congo and then Central African Republic, as the south moved closer to independence from Sudan.

South Sudan peacefully broke away from Sudan in 2011 after a decades-long fight for independence, giving Uganda a new sense of border security. Uganda, one of the South's strongest supporters in its quest for independence, denies it has taken sides in South Sudan's latest conflict, saying its forces provided security as Western countries and others safely evacuated their citizens from South Sudan.

Ugandan military spokesman Lt. Col. Paddy Ankunda insisted Monday that Ugandan forces are stationed only at the international airport in Juba and that their task is to "facilitate evacuation of civilians."  But United Nations workers in Juba told The Associated Press that Ugandan troops have been guarding the only bridge that crosses the Nile River.

Although Juba is now calm, unrest persists in other parts of the country.

Col. Philip Aguer, the South Sudanese military spokesman, said Monday that, although there was "no major fighting" over the weekend, tension remained because "Machar has not committed himself to a cease-fire. We've not seen one."

UN spokesman Martin Nesirky, however, said there was heavy fighting Sunday in Malakal, the capital of the oil-producing state Upper Nile. He said that while Kiir's government remains in control, the UN mission reported "significant battle damage in the city and widespread looting." He said 22,000 civilians have taken shelter in a UN compound there.

Pro-Machar forces still control Bentiu, the capital of South Sudan's other oil-producing state, Unity, and renegade troops are poised to attack Bor, the contested capital of Jonglei state, according to Aguer.

"There's a force advancing toward Bor," he said.

Machar has assured the U.N. representative to South Sudan, Hilde Johnson, that he is in control of the armed groups advancing toward Bor, according to Araud, the French ambassador, who spoke to reporters after the Security Council received a briefing from Johnson.

But Araud said the Security Council has "a lot of doubts" about Machar's claims. He said the armed youths are "one of the big question marks" of the conflict and "at the moment nobody really knows where it's heading or what is its strength."

He said that although both the government and Machar have stated their "availability for negotiations," both sides appear intent for now on gaining the upper hand militarily.

In Washington, US State Department deputy spokeswoman Marie Harf said Secretary of State John Kerry and other senior administration officials have been in regular contact with leaders in the region and in South Sudan to push for an immediate end to hostilities to allow for a mediated political dialogue and humanitarian access for citizens in need.

Harf said Kerry spoke with Kiir and Machar on December 24, 26 and 28, and he also also spoke with Kiir on December 20, 21 and 23.

"We have folks on the ground encouraging the parties to come and start negotiations in the coming days, but it's a very complicated, tenuous situation." Harf said. "Obviously, our first priority is keeping our folks safe, helping them leave the country."

She said the US so far has evacuated more than 400 US officials and private citizens and about 700 citizens from at least 27 other countries on seven chartered flights and six military aircraft.

The United Nations is scrambling to bolster its peacekeeping force in South Sudan from 8,000 troops and police to nearly 14,000, in part by transferring forces from missions in other vulnerable African countries.

Nesirky said 73 Bangladeshi police who had been serving in the U.N. Mission in Congo arrived in South Sudan last week and Nepalese police serving in Liberia were expected to arrive later this week.

But U.N. peacekeeping chief Herve Ladsous, who also briefed the Security Council on Monday, said the United Nations cannot meet its needs in South Sudan solely by transferring forces from other missions. He said the U.N. needs member states to contribute new troops and equipment, including a level 2 field hospital and tactical helicopters.

Although Kiir insists the latest unrest was sparked by a coup mounted by soldiers loyal to Machar late December 15,  this account has been disputed by some officials with the ruling party who say violence broke out when presidential guards from Kiir's majority Dinka tribe tried to disarm guards from the Nuer ethnic group of Machar.

South Sudan has been plagued by ethnic tension and a power struggle within the ruling party that appears to have escalated after Kiir sacked Machar as his deputy earlier this year. Machar has criticized Kiir as a dictator and says he will contest the 2015 presidential election.

The UN, South Sudan's government and other analysts say the dispute is political at its heart, but has since taken on ethnic overtones. The fighting has killed more than 1,000 people, according to the UN.

Muhumuza reported from Kampala, Uganda. Associated Press reporters Elias Meseret in Addis Ababa, Ethiopia, and Alexandra Olson at the United Nations contributed to this report.