Defense Secretary Chuck Hagel announced Thursday morning that the United States will be sending non-lethal aid to Ukraine.

The supplies will include medical aid, helmets, small fuel pumps, generators and tents, he said in a press briefing with Polish Defense Minister Tomasz Siemoniak.

"These measures are not meant to provoke or threaten Russia," Hagel said. "De-escalation has been our focus, and Russia must take steps to make that happen."

The U.S. has already sent other non-lethal aid to Ukraine's military, such as prepackaged meals.

The authorization of aid comes as tensions between Russia and the United States rise, with reports of a Russian fighter jet "buzzing" an American naval ship in the Black Sea.

"Our ship was in international waters. Nations have a right to defend themselves, and we take this right very seriously," Hagel said.

"Our military did raise this issue with the Russian military. We have many instruments in our government to deal with these things," the defense secretary said. But he did not elaborate on what kind of message America had sent Russia about the incident, or what was said.

With regard to Poland, a NATO ally, Hagel said that the United States was "fully committed" to its obligations with the alliance, and that options include sending ground forces to Poland on a rotational basis.

A planned American missile defense shield in Europe, which includes systems in Poland, has been criticized by Russian President Vladimir Putin as the start of an "arms race," a categorization Hagel strongly denied.

"President Putin says many things. That's ridiculous. It's not an arms race," he said at the press conference with the Polish defense minister. "The missile defense system in Europe is not a threat to Russia. It's a defense system."