The United States' ambassador to Nato, Nicholas Burns, has called on countries without troops tied up in Iraq to do more in Afghanistan.
Nato is now in control of peacekeeping in Afghanistan
Speaking as he arrived in Kabul for a meeting of the 26 member Nato council, Ambassador Burns said Afghanistan was the alliance's top priority.
He said he was confident that allied governments would provide the troops.
Officials hope that Nato will be operating in five more Afghan cities in the north and west by June.
'Wave of violence'
Mr Burns said that Turkey, Germany and other Nato allies without forces in Iraq - along with Spain which is pulling out its 1,300 troops - could help out with "resource problems" in Afghanistan.
"Nato needs to go faster and accelerate its build-up of troops in Afghanistan," he said.
He said that the alliance needed help from those countries which had "excess troop capacity".
Some Nato allies are reluctant to commit troops to Afghanistan
Correspondents say a recent wave of violence in the south and east of Afghanistan has underlined the security concerns.
Three US Marines were wounded, one seriously, over the week-end when a bomb exploded by their convoy in the southern province of Kandahar.
Senior American officers have expressed concern that Taleban and al-Qaeda elements may become more active during the spring.
"We have a significant number of groups in or coming in to Kabul with explosives and with specific missions to target groups, including us," Nato Lieutenant-General Rick Hillier said.
The alliance has agreed to set up and take command of five new teams, which will carry out small reconstruction projects or provide security for aid workers in the north and west by the end of June.
Spain's new government had sought to balance its Iraq pullout - strongly criticised by the Americans - by saying it was willing to provide troops in Afghanistan, but so far it has made no concrete offer.
Turkey, which has one of the largest military forces in the alliance, has declined to join the coalition trying to restore stability in post-war Iraq.
Correspondents say that the reluctance of the allies to offer resources for Afghanistan has dogged the credibility of Nato's first mission outside Europe or America in its 55-year history.
Nato is in charge of the 6,500 member International Stabilisation Force for Afghanistan - Isaf - which is drawn from 34 countries.
Another 1,500 American soldiers are fighting suspected Taleban and al-Qaeda guerrillas in the south and east of Afghanistan.