Two soldiers are missing in western Afghanistan after failing to return from a routine resupply mission two days ago, Nato officials have said.

"Exhaustive" search and rescue operations were being carried out to try to locate the soldiers, they said.

In a statement, Nato did not give the soldiers' nationalities or say which province they had been in when they went missing.

Military officials said the families of the two soldiers had been informed.

Nato added that three of its troops were killed in two separate roadside bomb attacks on Thursday. Two of the soldiers were American, but the nationality of the third was not given.

Correspondents say that reports of missing personnel in Afghanistan are extremely rare, although an American soldier has been missing in the south since late June.

Taliban insurgents in the area say they are holding him. US forces have been carrying out a widespread manhunt.

"We continue exhaustive search and rescue operations to locate our missing service members. We are doing everything we can to find them," a Nato press officer said of the latest incident.

"The families of these service members have been notified about their loved ones' status and we will continue to keep them informed as information becomes available."

Soldiers from more than 40 countries are taking part in Nato's force of nearly 110,000 troops in Afghanistan, two-thirds of them from the US.

The biggest contingents operating in the west of the country are from the United States and Italy.

Western Afghanistan has recently seen a rise in violence, with Taliban insurgents planting roadside bombs to maximise fatalities. This year more than 400 troops have been killed, most of them Americans.

US President Barack Obama is currently considering a request from the military to increase troop numbers by up to 40,000, a decision that is not likely to be made imminently.

The senior Nato commander in Afghanistan, Gen Stanley McChrystal, has warned that without an increase on the ground, the war could be lost.