The US military may send a warship to the Black Sea and take other steps to reassure anxious allies in Eastern Europe after Russia's intervention in Ukraine, a defense official said Tuesday.

"Some of the proposals that are being considered would be potentially putting another US warship in the Black Sea," a senior defense official told reporters travelling with Pentagon chief Chuck Hagel.

An American destroyer was deployed to the Black Sea earlier in the crisis over Ukraine but departed last month.

Commanders also were looking at expanding a scheduled exercise this summer involving troops from the US Army's 1st Cavalry division, said the official, who spoke on condition of anonymity.

A team of army officers might travel to Europe "soon" to start planning for a more large-scale exercise, the official said.

Hagel told reporters aboard his plane that while diplomacy remained Washington's focus to defuse the crisis, the United States and its NATO partners were looking at more measures designed to reassure alliance members on Russia's border.

In the aftermath of Russia's incursion in Crimea, governments have asked NATO's supreme commander, General Philip Breedlove, to present a list of possible options aimed at demonstrating the alliance's commitment to Eastern European members, including joint exercises, training and other steps, officials said.

"We've asked him (Breedlove) to look at the full range of measures," Hagel said.

But the Pentagon chief would not be drawn on whether the United States would consider permanently basing troops in Eastern Europe.

"We continue to look at different possibilities and options," he said.

Hagel said Washington and other NATO states had already taken steps "to reinforce those countries in Eastern Europe," by sending fighter jets for air patrols in Baltic countries and aviation training in Poland.

Although the United States and European governments may consider imposing additional economic sanctions against Russia over its actions in Ukraine, Hagel said "we want to de-escalate this."

But he said the crisis has unified NATO members and prompted a reaffirmation of their commitment to collective security, he said.

There has been a "coalescing of the NATO partners around the security commitments we have to each other and to Europe."

Hagel spoke en route to Hawaii for a meeting of ASEAN defense ministers, the first leg of a trip that will take him to Japan, China and Mongolia.