A first shipment of non-lethal goods to U.S. forces in Afghanistan is to leave soon from Latvia, an official said on Wednesday, as Washington seeks new supply routes in face of the closure of a base in Kyrgyzstan.

The United States needs alternative routes to supply its forces as Kyrgyzstan's parliament is to vote on Thursday on closing the Manas air base outside the capital Bishkek. One option to be used is by train from Riga, the Latvian capital.

"It is non-lethal commercial goods," said a spokesman for the U.S. embassy in Latvia. He said the shipment, of 100 containers, would leave "in the near future".

The embassy said 20 to 30 trainloads per week could go from Latvia to Afghanistan if the route proved a success. Latvia has been in NATO since 2004 and is a strong U.S. ally.

An official at the container terminal at Riga port said the shipment would leave very soon, but declined to give further details as it was a military cargo. He said the route would go via Russia and then through central Asian countries.

Washington is looking for alternative supply routes to landlocked Afghanistan because routes through Pakistan have become increasingly vulnerable to militant attacks.

Washington sent troops to Afghanistan after the attacks on the United States on Sept. 11, 2001.

They toppled the Taliban leadership for giving sanctuary to al Qaeda leaders responsible for the attacks.

U.S. President Barack Obama has ordered 17,000 more troops to Afghanistan to tackle an intensifying Taliban insurgency.

This will take U.S. troop numbers to about 55,000, joining 30,000 troops from 40 other mostly NATO states.

Kyrgyz President Kurmanbek Bakiyev, who needs parliamentary approval to shut the air base, announced the closure plans after accepting more than $2 billion in Russian aid and credit.