The United States will send helmets, medical supplies and other non-lethal military aid to Ukraine amid fears of another Russian incursion there, Defense Secretary Chuck Hagel said Thursday.

Hagel said he had informed Kiev that President Barack Obama "has approved additional non-lethal military assistance for health and welfare items and other supplies."

The aid includes medical supplies, helmets, sleeping mats and water purification units for Ukrainian troops, as well as shelters, small power generators and hand fuel pumps for Ukraine's border security force.

But it does not appear to contain any combat equipment, such as bulletproof vests or night-vision goggles.

Washington had sent some 300,000 food rations to Kiev in late March.

"The United States continues to stand with Ukraine," Hagel told reporters during a joint press conference with Poland's visiting Defense Minister Tomasz Siemoniak.

The aid makes up part of a long wish list of items sought by Kiev over the past several weeks, as its ill-equipped forces face a pro-Russian uprising in the east, and the specter of Moscow annexing another huge chunk of its territory after the seizure of Crimea last month.

"The United States will continue to review additional support that we can provide to Ukraine," Hagel said.

Several US lawmakers -- most notably veteran Republican Senator John McCain -- have issued scathing denunciations of the West's tepid response to the crisis.

McCain, who lost the White House election to Obama in 2008, decried steps taken by the United States and European Union thus far as "almost laughable" during a visit to Lithuania on Wednesday.

He and other lawmakers have called on Washington to provide military assistance to Ukraine, including light arms and anti-aircraft defenses.

Hagel tried to reassure NATO members such as the Baltic states and Poland worried about the impact of Russia's actions in Ukraine on their security.

Washington is "fully" committed to making sure their territory is respected, in accordance with NATO obligations, he said.

Since the start of the Ukrainian crisis, the Pentagon has sent 12 F-16 fighter jets and their support teams to Poland.

And Hagel said the planes would stay there until the end of the year, as he called on other European members of NATO to contribute reinforcements.

The United States and Poland separately agree to increase their military cooperation for anti-aircraft and special forces capabilities, according to Siemoniak.

Poland is due to host a NATO antimissile defense site in 2018 aimed at guarding against Iranian threats that have raised the ire of Moscow.

"We are talking about the presence of the American troops in Poland," the defense minister acknowledged.

But Siemoniak stopped short of calling for the establishment of US military bases, as requested by the conservative Polish opposition.

"There may be some new opportunities for rotational-basis forces," Hagel said.

"But no decisions have been made, and we'll continue to discuss these issues as NATO is discussing these issues."

Since the end of the Cold War, NATO has balked at setting up bases in members states that once belonged to the Soviet bloc, in an effort to avoid angering Russia.