NATO foreign ministers are due to discuss ways to help Ukraine and reassure allies in Eastern Europe, at a meeting in Brussels, BBC News reports.

It is the first time ministers from the 28 member states have convened since Russia's takeover of Crimea caused a diplomatic crisis.

NATO has bolstered annual air drills being held over Baltic countries later. Meanwhile, Russia has reportedly ordered a partial withdrawal of its troops from the border with Ukraine.

President Vladimir Putin told German Chancellor Angela Merkel he had ordered the move in a telephone conversation on Monday, March 31, according to the German government.

Thousands of Russian soldiers are still said to be deployed along the eastern border of Ukraine.

Meanwhile, Russian energy firm Gazprom is increasing the price it charges Ukraine for gas from Tuesday. Gazprom Alexei Miller said last month that Kiev had failed to pay its bills, believed to be more than $1.5bn.

NATO foreign ministers are expected to discuss the formal suspension of cooperation with Moscow at the Brussels meeting.

In a statement, the alliance said ministers would speak to Ukrainian Foreign Minister Andriy Deshchytsia about ways to support Ukraine with its defense reforms.

They are also expected to look at options including situating permanent military bases in the Baltic states to reassure members in Eastern Europe.

NATO jets will take part in air patrols in the region later in a routine exercise that analysts say has taken on added significance due to the crisis.

Several NATO countries, including the UK, U.S. and France, have offered additional warplanes.