WASHINGTON (REUTERS) - Republican presidential front runner Donald Trump is expected to be in Washington on Thursday (March 31) for a private meeting hosted by his top backer in the capital, US Senator Jeff Sessions of Alabama, three people familiar with the meeting said.

If it goes as planned, it will be Trump's second visit to Washington in two weeks. He was in Washington on March 21 for talks with Republican lawmakers in a meeting that also was hosted by Sessions.

Trump was expected to meet members of his foreign policy team, which is headed by Sessions.

"The meeting is taking place today in Washington," said Trump campaign manager Corey Lewandowski, without offering details.

Trump's meeting comes after he made a series of controversial statements about foreign policy in recent days that have raised concerns among Republican national security experts about where he would take the country if elected president on Nov 8.

In recent interviews, Trump has declared Nato obsolete, said Saudi Arabia is too dependent on the United States and said Japan and South Korea may need to develop their own nuclear programs because the US security umbrella is too costly to maintain.

In an MSNBC town hall on Wednesday night, Trump did not rule out the potential use of nuclear weapons in Europe or the Middle East to combat Islamic State militants. "I would never take any of my cards off the table," he said.

Max Boot, a conservative national security expert and member of the Council on Foreign Relations, wrote in Commentary Magazine this week that Trump is "singularly unqualified to be commander-in-chief".

"With Trump in command, our enemies would have a field day - Moscow and Beijing must be licking their chops at his desire to abandon US allies in Europe and Asia - and our friends would face mortal threats. If that isn't the single biggest threat to US security, I don't know what is," Boot wrote.