By Steve Holland, Kanishka Singh and Humeyra Pamuk

WASHINGTON (Reuters) -Ukrainian President Volodymyr Zelenskiy’s surprise visit to Washington, his first overseas trip since Russia invaded his country 300 days ago, started with a secretive train ride to Poland late on Tuesday.

The next morning, he arrived in the southern Polish city of Przemysl, where he was spotted at the train station according to footage from private broadcaster TVN, along with U.S. ambassador to Ukraine Bridget Brink, who accompanied him.

Zelenskiy’s visit had been planned for days and organized in secret because of concerns about his safety, but details were tweeted on Tuesday by a reporter from U.S.-based newsletter Punchbowl News.

In Poland, he boarded a U.S. government plane which landed at about noon EST (1700 GMT) on Wednesday at Joint Base Andrews in Maryland, some 12 miles southeast of Washington.

He traveled by motorcade to Blair House, the presidential guest house along Pennsylvania Avenue, and after freshening up from his trip, he traveled to the nearby White House, where he was greeted by President Joe Biden and his wife Jill Biden.

Rather than a suit and tie, Zelenskiy wore his signature army green sweater and cargo pants, a grim reminder that he came directly from a war zone.

There was no “mathematical formula” for the right day for Zelenskiy to make this trip, a senior Biden administration official told reporters late on Tuesday, but added that the trip would be “an important injection of momentum and sustenance” to U.S. and allied support as winter sets in.

The administration consulted closely with Zelenskiy on the “security parameters” of him being able to leave Ukraine and visit the United States briefly and return, said the official, speaking on the condition of anonymity.

“Of course, it ultimately was his decision to make. He concluded that those security parameters were met. What he needed, we agreed with that, and so we are executing accordingly,” the official said.

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Ukraine in recent weeks has come under intense Russian missile and drone bombardment to disrupt its energy infrastructure, leaving millions of people without electricity or running water in winter. Ukraine typically experiences long, cold winters with mean temperatures several degrees below zero Celsius and lows down to -20 Celsius.

Moments before Zelenskiy arrived, the U.S. State Department announced the latest package of security assistance to Ukraine worth $1.85 billion and including Patriot missiles, deemed to be one of the most advanced U.S. air defense systems.

At the White House, Zelenskiy held talks with Biden and his top national security aides, before participating in a joint news conference with Biden in the East Room, which was decked out with festive holiday decorations.

He then was proceeding to Capitol Hill to meet House of Representatives Speaker Nancy Pelosi and other lawmakers, and address a joint meeting of the U.S. Senate and House.

Zelenskiy’s less than 12-hour visit came together largely over the past two weeks after the two leaders discussed Zelenskiy coming over in a Dec. 11 phone call. The White House three days later extended a formal invitation to him, the U.S. official said.

Zelenskiy accepted the invitation last Friday and the visit was confirmed on Sunday.

At their news conference, Biden said it was important to have Zelenskiy visit “to look you in the eye.”

“President Zelenskiy, I’m honored to welcome you back to the White House,” said Biden.

(Reporting by Steve Holland, Kanishka Singh and Humeyra Pamuk; Editing by Kieran Murray and Grant McCool)

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