WASHINGTON (Reuters) - U.S. Secretary of State Rex Tillerson will press this week for NATO allies to demonstrate a “clear path” to increase defense spending, a State Department official said on Tuesday.

U.S. Secretary of State Rex Tillerson is seen at the Department of State in Washington, U.S. March 28, 2017. REUTERS/Eric Thayer

Tillerson will hold his first meeting with NATO foreign ministers in Brussels on March 31.

He will push allies on how they plan to meet a defense spending goal of 2 percent of gross domestic product, and press NATO to increase its role in the fight against terrorism, the official said.

“It is no longer sustainable for the United States to maintain a disproportionate share of NATO’s deterrence and defense spending,” the official said in a briefing with reporters, on condition of anonymity.

President Donald Trump has unsettled European allies with demands they increase defense spending and talk of establishing an alliance with Russia to counter Islamic State militants.

Tillerson’s initial decision to skip his first meeting with NATO foreign ministers to attend expected talks in the United States with Chinese President Xi Jinping also reopened questions about the Trump administration’s commitment to the alliance. The State Department later said the meeting in Brussels had been rescheduled and Tillerson would attend.

Five NATO members - Britain, Estonia, Greece, Poland and the United States - currently meet the 2 percent spending threshold, according to 2016 NATO figures. Members of the alliance have until 2024 to meet the targets.

The Estonian, Latvian, and Lithuanian foreign ministers met with Tillerson at the State Department on Tuesday. The Baltic states have felt especially vulnerable since Russia annexed Ukraine’s Crimea region in 2014.

Asked if they were confident in U.S. support for NATO, Lithuanian Foreign Minister Linas Linkevicius responded “No doubts about that” and Latvian Foreign Minister Edgars Rinkevics and Estonian Foreign Minister Sven Mikser nodded in agreement.

The senior State Department official said Trump administration officials are “pushing allies to do more, faster, absolutely no apology for that.” The United States also wants allies to give a “clear path” on how they would meet the threshold, such as timelines and budgetary commitments, he said.

But the official declined to state any specifics on what the United States would do if allies did not meet the targets.

“Our joint security requires it, that’s the main leverage that we have,” the official said.

Defense Secretary Jim Mattis said last month the United States might “moderate” its support for the alliance but gave no details.