Three rival Pakistani Taliban groups have agreed to form a united front against international forces in Afghanistan in a move likely to intensify the insurgency just as thousands of extra US soldiers begin pouring into the country as part of Barack Obama's surge plan.

The Guardian has learned that three of the most powerful warlords in the region have settled their differences and come together under a grouping calling itself Shura Ittihad-ul-Mujahideen, or Council of United Holy Warriors.

Nato officers fear that the new extremist partnership in Waziristan, Pakistan's tribal area, will significantly increase the cross-border influx of fighters and suicide bombers - a move that could undermine the US president's Afghanistan strategy before it is formulated.

The unity among the militants comes after a call by Mullah Omar, the cleric who leads the Afghan Taliban, telling Pakistani militants to stop fighting at home in order to join the battle to "liberate Afghanistan from the occupation forces".

The Pakistani Taliban movement was split between a powerful group led by the warlord Baitullah Mehsud and his bitter rivals, Maulvi Nazir and Gul Bahadur. While Mehsud has targeted Pakistan itself in a campaign of violence and is accused of being behind the assassination of the former prime minister Benazir Bhutto, Nazir and Bahadur sent men to fight alongside other insurgents in Afghanistan.

The move potentially provides short-term relief in Pakistan but imperils Nato forces, especially those stationed in southern and eastern Afghanistan, including the British, close to the Pakistani border.

"It's of concern to us when we see a grouping like that," said a western security official in Pakistan. "This can't be ignored."

Fears of an increase in fighting come as the International Committee of the Red Cross (ICRC) warned yesterday that civilians would face the brunt of any increase in violence in Afghanistan. Ordinary Afghans were now more at risk from the fighting than at any time since the start of the war in 2001, said Pierre Kraehenbuehl, director of operations for the ICRC.

Violence in Afghanistan intensified last year with some 5,000 people killed, including more than 2,100 civilians, a 40% increase on the previous year, the UN reported last month.

Pakistan was already under intense western pressure to act against extremists based in its tribal area. A western military adviser, also based in Pakistan, said a Pakistani Taliban alliance would cement the grip of the militants over Waziristan. The region is also home to Afghan Taliban and al-Qaida, who use Waziristan and other parts of the tribal area as a haven to regroup and launch attacks against Afghan and Nato forces.

"No insurgency has ever been destroyed as long as the sanctuaries are still alive. If the sanctuaries are gaining more strength, that certainly worries Nato," said the military adviser.

The Obama administration in Washington has announced 17,000 extra troops for Afghanistan. American forces will concentrate on areas close to the Pakistani border, which are seen as the most troublesome. Obama is pressing European countries to also boost their troop numbers.

In an apparent response to the augmented US challenge, Mullah Omar has directed Pakistani militants in Waziristan to halt attacks on Pakistani forces.Baitullah Mehsud is feared in Pakistan, having led an assault on his own country since 2007, killing hundreds of soldiers, policemen and ordinary Pakistanis through suicide attacks and other bombings. But his tactics, influenced by al-Qaida, were controversial even within the Taliban.

"If anybody really wants to wage jihad, he must fight the occupation forces inside Afghanistan," Mullah Omar told Pakistani militants in a letter. "Attacks on the Pakistani security forces and killing of fellow Muslims by the militants in the tribal areas and elsewhere in Pakistan is bringing a bad name to mujahideen and harming the war against the US and Nato forces in Afghanistan."

The Pakistani Taliban recognise Mullah Omar, founder of the Taliban movement in Afghanistan, as their ultimate leader, although operationally they work independently.

"Baitullah Mehsud is now taking on the Americans," said Talat Masood, a retired Pakistani general turned analyst. Baitullah Mehsud has recently called off his fighters in two key battles inside Pakistan, with ceasefires declared in Swat valley, in the North West Frontier Province, and Bajaur, another tribal area. While Pakistani forces claim to have "won" in Bajaur, they show no appetite for taking the war to Waziristan.

Controversially, the Pakistani government has acceded to the militants' demand for Islamic law in Swat. Under two secret peace deals signed by Pakistani authorities with the militants last year, covering north and south Waziristan, a truce exists there.

While western countries want to see the Pakistani army take the fight to Waziristan, Pakistani forces have been repeatedly defeated there. Major General Athar Abbas, chief spokesman for the Pakistan army, said that there was "no plan" to start operations in Waziristan. "It's the government that decides these things," he added.