KABUL - The United Nations in Afghanistan has issued a plea for all sides to avoid civilian casualties after the Taliban declared Sunday would be the start of a stepped-up campaign of violence across the country that would include suicide attacks.

The hardline Islamists have warned civilians to stay away from public gatherings, military bases and convoys, as well as government centres and buildings, as these would be the focus of a wave of attacks beginning on Sunday.

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"Parties to the conflict must not deliberately attack, target or kill civilians, or indiscriminately harm them," said Staffan de Mistura, the UN chief in Afghanistan, in a statement released late on Saturday.

"We call on all parties to take all possible measures to  protect civilians, especially in the forthcoming months when we expect, unfortunately, intensified conflict," he said.

Senior military commanders have been expecting a spike in violence with the arrival of the spring and summer "fighting season", although the usual winter lull was not seen as US-led forces pressed their attacks against insurgents, particularly in the Taliban's southern heartland.  

Senior military officials say recent intelligence reports indicate the fresh campaign of increased violence will last about a week and be mounted by the Taliban, supported by the al Qaeda-linked Haqqani network and other insurgents.

Security has been increased at military bases and government offices, while in Kabul extra police have been stationed at so-called ring of steel security checkpoints around the city to search vehicles.

The Taliban said in a statement on Saturday the targets of the attacks would be foreign forces, high-ranking officials of President Hamid Karzai's government, members of the cabinet and lawmakers, as well as the heads of companies working for the NATO-led International Security Assistance Force (ISAF).

While Washington and ISAF commanders believe they have made inroads against a growing insurgency since 30,000 extra US troops were sent to Afghanistan last year, the violence has shown little sign of abating.  

Attacks across Afghanistan hit record levels in 2010, with civilian and military casualties the worst since US-backed Afghan forces toppled the Taliban government in late 2001.

The United Nations said it had relocated some of its staff in Afghanistan after receiving what it said were credible threats of increased attacks in several locations around the country.  

The United Nations has been the target of several insurgent attacks over the past two years and seven international staff were killed last month when protesters overran a UN compound in the northern city of Mazar-i-Sharif.

De Mistura said indiscriminate use of bombs by the Taliban in cities and elsewhere had caused huge numbers of civilian casualties, while air strikes by the NATO-led force had also caused many deaths.

The number of civilians killed in Afghanistan in 2010 rose 15 percent from the previous year to 2,777, according to the United Nations, with insurgents responsible for about three-quarters of those deaths.

"Afghan civilians have paid the price of war for too long - it is more urgent than ever that all parties act to prevent this suffering and that in the forthcoming spring we also see a surge in protection of civilians," de Mistura said.