KABUL, Afghanistan - US officials in Kabul said on Monday the movements of staff in parts of Afghanistan's volatile south were being restricted, warning of more attacks after a two-day siege came to a bloody end and insurgents killed at least 11 people in other attacks.

The US Embassy in Kabul issued a security bulletin in which it said it had received specific threats of attacks in three areas in Helmand province. It gave no details about the nature of the threats.

Helmand lies west of Kandahar, the birthplace of the Taliban and the focus of efforts by tens of thousands of US, NATO and Afghan troops to quell a growing insurgency over the past year.

Afghan troops, aided by NATO-led forces, on Monday mopped up the remnants of a major assault launched by the Taliban in Kandahar city, the main city in the south, where the governor's compound and other key facilities were attacked by suicide bombers and Taliban fighters on Saturday.

"US government personnel in Marjah have been confined to their compounds due to a reported specific threat to Afghan government facilities in Marjah, Lashkar Gah and possibly Gereshk beginning today," the US bulletin said.

Lashkar Gah, the capital of Helmand, is one of seven areas where a gradual handover of responsibility from foreign troops to Afghan security forces will begin in July.

That handover is the first stage of a plan under which all foreign combat troops will leave by the end of 2014.

Helmand has seen some of the worst fighting in the near decade-long war against the Taliban and other Islamist insurgents. In 2010, thousands of US Marines and Afghan forces assaulted Marjah to clear insurgent strongholds, but ran into fierce resistance.  

Spring offensive

The Taliban declared this month the start of a new "spring offensive". Dozens of insurgents had battled Afghan forces in Kandahar city since Saturday, holing up in a hotel and shopping mall before the last were killed on Sunday.

The battle paralysed the city, with streets and shops closed as gunfire and explosions sent panicked residents fleeing.  

Kandahar provincial governor Tooryalai Wesa said at least 20 attackers, many of them suicide bombers who had used explosives-packed vehicles, were killed during the operation.

Three Afghan troops and a civilian were also killed in battle that showed the Taliban retain the ability to launch telling strikes in an area where US and Afghan leaders say significant progress has been made against the insurgents.

Wesa said 40 civilians and police were wounded.

Security officials took media to a building near Afghan intelligence offices in Kandahar where the last insurgents held off troops and police for more than 40 hours. Their bodies still lay inside the partially destroyed five-storey structure.

In the east, three Afghan civilians were killed by a suicide bomber on a motorcycle targeting a convoy of foreign troops in the Qarghayo district of Laghman province, district governor Saleh Mohammad said. The Taliban claimed responsibility.

A spokesman for the NATO-led International Security Assistance Force (ISAF) said some troops were also wounded but gave no details. Saleh said 11 people were wounded.

Four villagers were found beheaded in eastern Khost province, although the Taliban denied responsibility, local authorities said. Taliban insurgents ambushed and killed four Afghan police in central Ghazni province on Sunday, police said.

Violence across Afghanistan last year reached its worst levels since the Taliban were overthrown in late 2001, with record casualties on all sides of the conflict.

The Taliban have managed to carry out a number of high-profile attacks inside Kandahar and in the capital Kabul over the past year despite Afghan and foreign forces beefing up security around both cities.