WASHINGTON, May 21 (Ross Colvin/Reuters) - President Barack Obama is likely to raise US concerns about Syria arming Hezbollah guerrillas in Lebanon when he meets Lebanese Prime Minister Saad al-Hariri on Monday, a US official said on Friday.

Hariri's first official visit to the United States takes place against a backdrop of tensions in the Middle East, US efforts to restart Israeli-Palestinian peace talks and growing momentum toward new international sanctions on Iran.

Analysts expect Obama to be more encouraging in tone than demanding of results when he meets Hariri, who heads a national unity government that includes Hezbollah -- a Shi'ite Islamist guerrilla group which is backed by Syria and Iran and is listed as a terrorist organization by the United States.

White House spokesman Robert Gibbs said on Friday the two leaders would discuss a broad range of mutual goals in support of Lebanon's sovereignty and independence, regional peace and security.

Lebanon and Syria have said they fear a possible attack by the Jewish state after its president, Shimon Peres, accused Syria in April of supplying Hezbollah with long-range Scud missiles capable of hitting Israel. Damascus has denied the charge and accused Israel of fomenting war.

Some U.S. officials have expressed doubt that any Scuds were actually handed over in full to Hezbollah, although they believe Syria might have transferred weapons parts.

We obviously have grave concerns about the transfer of any missile capability to Hezbollah through Lebanon from Syria, a senior Obama administration official told Reuters, saying the issue would likely be raised in Monday's talks.

Another official said Washington would ask Hariri to continue to support efforts toward comprehensive regional peace.

LEBANON'S CHANGING ROLE

Hariri has also denied Israel's accusations, while his government has said it backs the right of the guerrilla group to keep its weapons to deter Israeli attacks. Israel, which fought a 34-day war with Hezbollah in 2006, has not signaled any imminent plans to strike.

The war of words heightened tensions in the region, but the U.N. Special Coordinator for Lebanon, Michael Williams, noted on Friday that recent tension is now diminishing.

Williams, who held talks with Hariri in Beirut, was quoted by the prime minister's office as saying he was pleased that all sides have scaled back the rhetoric.

Obama and Hariri are also expected to discuss U.S.-led international efforts to isolate Iran over its disputed nuclear program, officials said. Lebanon holds the rotating presidency of the U.N. Security Council through May 31.

Diplomats said Beirut had quietly asked the permanent members of the Security Council -- Britain, France, Russia, China and the United States -- not to push for a vote on a new Iran sanctions resolution while it held the presidency.

Lebanon is expected to abstain in any vote because Iranian-backed Hezbollah is in its government, diplomats said.

Jon Alterman, a Middle East expert at the Center for Strategic and International Studies in Washington, said Lebanon no longer enjoyed the status it had under the Bush administration, when it was the fulcrum of efforts to spread democracy in the Middle East.

The Obama administration's Middle East policy is more focused on the nuclear stand-off with Iran, war in Afghanistan, and reviving the Middle East peace process, he said.

Nevertheless, the United States has expanded military assistance to Lebanon to strengthen its armed forces as a counterweight to Hezbollah, allocating $500 million to training and equipping Lebanese security forces since 2005. (Additional reporting by Louis Charbonneau in New York; editing by Mohammad Zargham)