Six hundred U.S. troops are headed for Poland and the three Baltic countries for live-ammunition infantry exercises with armed forces from the four former Soviet-bloc nations.

Adm. John Kirby, the Pentagon’s press secretary, said Tuesday that the dispatch of troops from U.S. bases in Italy and Germany for military maneuvers near Russia’s western border is in response to its aggression in Ukraine. “If there’s a message to Moscow, it is the exact same message [as delivered before], that we take our obligations very, very seriously,” Kirby told reporters at the Pentagon.

The U.S. troops will be in Poland, Latvia, Estonia and Lithuania by Monday and more U.S. forces could be sent to other NATO nations, Kirby said.

“These exercises were conceived and added to the [regular] exercise regime as a result of what’s going on in Ukraine,” Kirby said.

He said the first 600 troops, which will be drawn from the 173rd Infantry Brigade Combat Team (Airborne) headquartered in Italy, will conduct monthlong exercises with about 150 airborne ­soldiers assigned to each of the four countries. That first group will be replaced in a month by a similar-sized contingent divided among the four nations, with subsequent rotations planned each month for the rest of the year.

“What we’re after here is a persistent presence — a persistent rotational presence in these exercises,” Kirby said.

Russian troops began seizing control of Crimea, a peninsula in southeastern Ukraine with a Russian naval port, early last month. Russia claims to have annexed Crimea, which has a majority ethnic-Russian population, as a result of a March 16 referendum. The United States, along with many other governments and the United Nations, has not recognized the results of the referendum.

The United States last month stepped up its participation in NATO air patrols over the Baltics and in NATO training of Polish military pilots. But Kirby said the new troop movements are U.S.-bilateral exercises beyond the auspices of the transatlantic military alliance.

Meanwhile, Vice President Joe Biden on Tuesday met with Ukrainian Prime Minister Arseniy Yatsenyuk and Ukrainian legislators during a visit to the former Soviet republic. “The road ahead is difficult,” Biden said at a news conference with Yatsenyuk in the Ukrainian capital of Kiev. “But you will not walk this road alone. We will walk it with you.”

Biden called on Russia to pull back thousands of troops massed on the Ukrainian border.