US Secretary of State John Kerry has called on Syrian President Bashar al-Assad to make "a commitment to find peace in his country," ahead of a meeting of the so-called Friends of Syria group in Amman.
Speaking at a news conference with his Jordanian counterpart, Nasser Judeh, Kerry said a US-Russian proposed conference in Geneva planned for next month seeks "to end the bloodshed that has cost tens of thousands of lives".
"We would call on President Assad to make the same commitment to find peace in his country," Kerry said.
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The foreign ministers of Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States are attending the Amman gathering.
The meeting seeks to discuss a US-Russian proposal to hold a peace conference to bring together rebels and representatives of Assad's regime.
As Kerry and his counterparts arrived at the meeting venue in Amman, about 250 pro-Assad demonstrators blocked the main entrance.
The protesters, a mix of Jordanians and Syrians, chanted "Death to America," and, "Go home, Kerry we don't want you here."
Kerry said that if the Assad regime is unwilling to negotiate a political solution, the US and others will discuss growing support for the opposition "in order for it to continue to fight for the freedom of their country".
He said several thousand Hezbollah fighters were taking part in the Syrian conflict with active Iranian support on the ground.
"Just last week, obviously, Hezbollah intervened very, very significantly," the US Secretary of State said.
Also speaking in Amman, British Foreign Secretary William Hague said Iran and its Lebanese ally Hezbollah are "propping up" Assad and giving him increasing support.
"It is very clear that Syrian regime is receiving a great deal of support, increasing support in recent months from outside Syria from Hezbollah and Iran," Hague said.
"The regime is being propped up by others outside, further undermining its legitimacy. It also shows that is a crisis that is increasing the threat to regional stability."
Hague's comments came as Syria's main opposition bloc urged rebels to come from around the country to reinforce a town under attack by government troops and Hezbollah fighters.
Dozens of Hezbollah members have been killed fighting in the town of Qusayr and the surrounding areas in the past weeks.
The battle for Qusayr has now entered its fourth day and acting chief of the Syrian National Coalition, George Sabra, said that "forces from outside Syria" aim to destroy the town and rebels should join the fight to "rescue" it.
Sabra also urged Lebanese authorities to respect Syria's sovereignty by preventing foreign gunmen from crossing the border to fight in the civil war.
Kerry also condemned Hezbollah's involvement in Syria, saying "several thousand" fighters from the group were taking part in the conflict.
Syria's ambassador to Jordan, Bahjat Suleiman, denounced the Amman meeting, calling it part of a US-Israeli campaign to destroy his country.
He castigated Arab nations for attending, saying they are aiding Washington in its "aggressive, terrorist war, on behalf of Israel, to destroy Syria".