Romania has approved an increase in American troops at its military airbase on the Black Sea as Washington continues to shift its main transit base for Afghanistan away from Kyrgyzstan, a report said Tuesday.

Romanian President Traian Basescu sent a letter to parliament approving the US request for 600 more troops at the Mihail Kogalniceanu airport in the southeast of the country, the Agerpres news agency said.

"The request... refers to the deployment of a unit of Marines called the Special Purpose Marine Air-Ground Task Force Crisis Response, with up to 600 soldiers and a number of military aircraft needed to fulfil specific missions," the letter said.

The US is due to abandon by July the Manas airbase in Kyrgyzstan that has been the main transit point for its military personnel and cargo in and out of Afghanistan, after the Kyrgyz government refused to extend the lease last year.

The Mihail Kogalniceanu base, where around 1,000 troops are currently stationed, is due to become the main transit point as the US prepares for a major withdrawal of forces and equipment from Afghanistan later this year.

The US inaugurated the new transit centre in February.

Between 20,000 and 30,000 troops are expected to transfer through the base on their way back from Afghanistan this year.