The United States has not determined whether Syria has transferred Scud missiles to Lebanon's Hezbollah, a U.S. State Department spokesman said on Tuesday.

"We are still looking into it. We haven't [made] any particular judgment at this point as to whether any transfer has taken place but ... this is something that we have great concern about," Assistant Secretary of State P.J. Crowley told reporters.

The United States summoned the senior Syrian diplomat in Washington on Monday to address what it called "provocative behavior" regarding the potential transfer of Scud missiles to Hezbollah, which it said could be threaten Lebanon and Israel.

"The United States condemns in the strongest terms the transfer of any arms, and especially ballistic missile systems such as the Scud, from Syria to Hezbollah," the statement, issued by State Department spokesman Gordon Duguid, said.

"The transfer of these arms can only have a destabilizing effect on the region and would pose an immediate threat to both the security of Israel and the sovereignty of Lebanon."

The U.S. statement on Monday stopped short of confirming the alleged transfer of long-range Scud missiles to Lebanese Hezbollah guerrillas, which if true could cast doubt on U.S. President Barack Obama's diplomatic outreach to Syria.

Lebanese PM: False Israeli Scud accusations are like charges Iraq had WMDs

Earlier Tuesday, Lebanon's Western-backed prime minister Saad Hariri denied Israeli allegations that Hezbollah has obtained Scud missiles, comparing them to the false American charges that Iraq had weapons of mass destruction ahead of the 2003 U.S.-led invasion.

Scud missiles, which can carry a warhead of up to 1 ton, would be far larger than the biggest rockets previously in Hezbollah's arsenal.

Syria has denied the Israeli allegations and Hezbollah has neither confirmed nor denied them.

Hariri compared the charges with U.S. allegations that Saddam Hussein was developing weapons of mass destruction, which were the main rationale behind the U.S.-led war in Iraq but turned out to be false.

"The media suddenly started reporting that there are Scud missiles in Lebanon. Do you know what a Scud missile means? I believe it is as big as this room," Hariri said late Monday in Italy. His comments were reported by Lebanese media Tuesday. "Threats that Lebanon now has huge missiles are similar to what they used to say about weapons of mass destruction in Iraq," he added. "These are weapons that they did not find and they are still searching for."

"They are trying to repeat the same scenario with Lebanon," the prime minister said.

The allegations dealt a new setback to long-tense Syrian-U.S. relations, which had begun to improve after U.S. President Barack Obama took office last year. Earlier this year, Washington named a new ambassador to Damascus five years after withdrawing its top diplomat from Syria.

In Syria, Foreign Ministry spokeswoman Bushra Kanafani repeated Damascus' denial and said Syria was surprised the United States was adopting Israeli claims.

"This is regrettable and could deepen the gap between Damascus and Washington," Kanafani said, adding that Israel is trying to shuffle the cards in the region because of its foreign relations dilemma over its settlement policies in Palestinian territories.

Last week, White House press secretary Robert Gibbs told reporters that the U.S. had raised its concerns about reports of Syrian Scud missile transfers to Hezbollah with the highest levels of the Syrian government.

Hezbollah Minister Hussein Haj Hassan refused to confirm or deny Israeli allegations that the militant group has acquired Scud missiles. He said Friday the group was always arming itself but, "what we have is not their business."

Hezbollah's leader, Sheik Hassan Nasrallah, has said his militants have more than 30,000 rockets and are capable of hitting anywhere in Israel. Those claims match Israeli intelligence assessments.

Hezbollah and Israel fought a 34-day war in 2006 that left some 1,200 Lebanese and 160 Israelis dead. During the month-long conflict, Hezbollah fired nearly 4,000 rockets at northern Israel, including several medium-range missiles that for the first time hit Israel's third-largest city, Haifa.