Up to 150 U.S. troops and military equipment will be arriving in Bulgaria this week for joint training amid escalating tensions between Russia and Ukraine, the defense ministry said on Tuesday.

On Monday Russian President Vladimir Putin recognized two breakaway regions in eastern Ukraine and has since ordered the deployment of troops there, deepening Western fears of a major war in Europe.

The U.S. soldiers and armored vehicles will be crossing from into Bulgaria from Romania on Tuesday and Thursday to take part in the training aimed at boosting troops' interoperability, the ministry said in a statement.

Under a deal with NATO ally Bulgaria, the United States can keep up to 2,500 U.S. troops at several joint bases in the Black Sea country. Last week Defence Minister Stefan Yanev said a U.S. army Stryker company will be at Novo Selo military base in eastern Bulgaria for joint training.

Bulgaria, which joined NATO in 2004, plans to lead a battlegroup of up to 1,000 troops on its territory as NATO moves to strengthen its deterrence and defense on its eastern flank.

Spain has deployed four Eurofighter jets to Bulgaria to support it in policing its air space until the end of March.