WASHINGTON, May 24 (Matt Spetalnick and Ross Colvin/Reuters) - Lebanon's prime minister warned President Barack Obama on Monday of pervasive Arab frustration with Middle East peace efforts, and the two leaders discussed US concerns that Syria may be arming Hezbollah guerrillas.

Prime Minister Saad al-Hariri's first official US visit took place against a backdrop of tensions in the Middle East, US efforts to restart the Israeli-Palestinian peace process and growing momentum toward new US sanctions on Iran.

Analysts expected Obama to be more encouraging in tone than demanding of results with 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.

Speaking after the White House meeting, Hariri praised Obama for seeking to revive Israeli-Arab peace diplomacy, but said he told the president the clock is ticking, and it is ticking against us.

I also pointed to the pervasive frustration and skepticism in the Arab and Muslim worlds regarding this issue, Hariri told reporters. Have no doubt. Failure will nurture more extremism and give birth to new forms of violence.

Israel and the Palestinians launched indirect, US-brokered talks earlier this month, but broad differences remain and neither side is optimistic of a breakthrough soon.

Arab states have largely resisted the Obama administration's appeals for goodwill gestures toward the Jewish state. Seeking progress on Middle East peace is a centerpiece of Obama's outreach to the Muslim world.

Asked whether he and Obama had talked about US concerns that Syria may have sent missile parts to Hezbollah, Hariri said, We discussed all these issues. Obama, apparently trying to keep the visit low-profile, did not appear in public with the Lebanese leader.

A White House statement issued after the meeting said Obama had discussed with Hariri the threat posed by the transfer of weapons into Lebanon in violation of a UN resolution that helped end the 2006 Israel-Lebanon war.

U.S. DOUBTS

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 deep inside Israel. Damascus has denied the charge and accused Israel of fomenting war.

Some US 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 on Friday.

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.

Obama, who is spearheading a push for tighter UN sanctions on Iran, also used his meeting with Hariri to try to shore up Arab opposition to Tehran's nuclear ambitions.

The president stressed the importance of efforts to ensure Iran complies with its international nonproliferation obligations, the White House said.

Lebanon holds the rotating presidency of the UN Security Council through May 31. Diplomats said Beirut had quietly asked the permanent members of the council not to push for a vote on a new Iran sanctions resolution while it held the presidency.

Hariri, who has sought to reduce Syrian influence in Lebanon, urged Obama to continue US support to build up Lebanon's security forces. Hariri and his political allies accused Syrian of complicity in the assassination of his father and former prime minister, Rafik al-Hariri, in 2005.

Washington sees the Lebanese army as an important counterweight to Hezbollah. (Additional reporting by Alister Bull; Editing by Eric Walsh)