U.S. Defence Secretary Jim Mattis briefs the media during a NATO defence ministers meeting at the Alliance's headquarters in Brussels, Belgium February 15, 2017. REUTERS/Francois Lenoir

SOFIA (Reuters) - U.S. troops deployed in Bulgaria on Wednesday and armored vehicles and heavy equipment are to arrive by the end of the week under a planned NATO operation to support its Eastern European allies, the Bulgarian defense ministry said.

The U.S. deployment at the joint U.S.-Bulgarian military base at Novo Selo in the east of the Black Sea country is part of operation Atlantic Resolve, aimed at showing Moscow Washington’s commitment to its allies, the ministry said.

In moves agreed last year under former U.S. President Barack Obama, NATO is expanding its presence in the region to levels unprecedented since the Cold War, prompted by Russia’s annexation of Crimea and accusations - denied by Moscow - that it is supporting a separatist conflict in eastern Ukraine.

Jim Mattis, on his debut trip to Europe as U.S. defense secretary, said on Wednesday President Donald Trump firmly backed NATO. As a candidate, Trump fiercely criticized the alliance and appeared to question its value.

But Mattis warned NATO allies in Brussels they must honor military spending pledges to ensure the United States did not “moderate” its support for the alliance.

Bulgaria’s Defence Ministry said up to 120 U.S. troops were accommodated at the Novo Selo training range, with a company of the 3rd armored combat team from Fort Carson, Colorado, being deployed.

“Joint drills and training at Novo Selo training range will be increased this year. The U.S. army troops will be rotated for the drills,” it said.

The U.S. deployment to NATO’s eastern flank includes more than 80 main battle tanks and hundreds of armored vehicles. The military unit will rotate through several countries, including Bulgaria, Estonia, Latvia, Lithuania and Romania.

As part of the largest U.S. military reinforcement of Europe in decades, around 2,700 troops, out of 3,500 planned, arrived in Poland in January, vexing the Kremlin, which said the troops’ presence is a threat to Russia.