Under fire at home, Trump wins warm welcome in Saudi

* King greets Trump and wife as they step off plane

* Defence and energy deals worth tens of billions

* Salman overheard lamenting plight of Syria (Adds Trump quotes)

By Jeff Mason and Steve Holland

RIYADH, May 20 (Reuters) - Dogged by controversy at home, U.S. President Donald Trump won a warm reception in Saudi Arabia on Saturday but struggled to shift attention from the political firestorm over his firing of former FBI Director James Comey.

A $110 billion deal in which Saudi Arabia will buy U.S. arms to help it counter Iran, with options running as high as $350 billion over 10 years, was the central achievement of Trump's first day in Riyadh, first stop on a nine-day journey through the Middle East and Europe.

Speaking to journalists after a ceremony to exchange agreements, Trump said it was a "tremendous day" and expressed his thanks to Saudi Arabia.

"Tremendous investments in ... the United States, and our military community is very happy," he said.

"Hundreds of billions of dollars of investments into the United States and jobs, jobs, jobs. So I would like to thank all of the people of Saudi Arabia."

But the turmoil back home consumed the headlines and cast a long shadow over his first foreign trip as president.

His firing of Comey and the appointment of a special counsel to investigate his election campaign's ties to Russia last year have raised the question of whether he tried to squelch a probe into the alleged Russia connection.

Fanning the flames was a New York Times report that Trump had called Comey a "nut job" in a private meeting last week in the Oval Office with Russian Foreign Minister Sergei Lavrov and ambassador Sergei Kislyak. The Times quoted briefing notes of the conversation.

Asked for a response, the White House said that for national security reasons, "we do not confirm or deny the authenticity of allegedly leaked classified documents."

Russia's Interfax news agency on Saturday quoted Lavrov as saying he had not discussed Comey with Trump. "We did not touch this issue at all," the minister said.

In another development, the Washington Post said a current White House official close to Trump was a significant "person of interest" in the investigation into possible ties with Russia.

The fallout followed Trump to Riyadh but did nothing to cool the welcome he received by the royal Saudi family.

King Salman bin Abdulaziz greeted Trump on a red carpet as he stepped off Air Force One, shaking the hand of his wife, Melania, and riding in the U.S. presidential limousine.

CONTRAST WITH OBAMA VISIT

It was a more favorable welcome than had been granted last year to Trump's predecessor, Barack Obama, who was seen in the Arab kingdom as soft on Iran and hesitant on Syria.