The United States and Gulf Arab states on Saturday urged envoy Kofi Annan to produce a "timeline for next steps" in his peace plan for Syria if President Bashar al-Assad fails to stop the bloodshed.
US Secretary of State Hillary Clinton, who met her counterparts from Saudi Arabia, Kuwait, Bahrain, Qatar, the United Arab Emirates and Oman at a meeting in Riyadh, voiced concern over Syria's continued deadly crackdown on dissent.
She also slammed Iran for its alleged support for the crackdown and appeared cautious about talks between Tehran and the United States and five other powers that she confirmed would be held in Istanbul on April 13.
In a press conference, Clinton voiced renewed scepticism about Syria's acceptance of Annan's six-point peace plan, which calls on Syrian forces to withdraw from besieged cities and silence their guns.
"And as of today, regime forces continue to shell civilians, lay siege to neighbourhoods, and even target places of worship," Clinton said on the eve of international talks in Istanbul aimed at helping the Syrian opposition.
Clinton said dozens of top Arab and Western officials on Sunday would discuss further steps to pressure Assad, to provide humanitarian aid and promote "an inclusive, democratic" political transition.
The first US strategic cooperation forum with the six Gulf Cooperation Council (GCC) countries issued a joint statement urging Annan "to determine a timeline for next steps if the killing continues."
Annan's plan calls for a commitment to stop all armed violence, a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate, and release of arbitrarily detained people.
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In a speech to the forum, Clinton stressed Washington's "rock solid and unwavering" commitment to the security of the Sunni Muslim-led Gulf Arab monarchies, which are wary of non-Arab Shiite Muslim Iran.
Raising security ties from a bilateral to a multilateral level, Clinton was breaking new ground in taking part in the first strategic cooperation forum between Washington and the GCC.
She looked to taking "practical and specific steps to strengthen our mutual security, such as helping our militaries improve interoperability, cooperate on maritime security and missile defence, and coordinate responses to crises."
US officials have said it is a US "priority" to help the GCC build a "regional missile defence architecture" against what they see as a looming ballistic missile threat from Iran.
Speaking at the press conference, the chief US diplomat broadened her attack on Tehran.
Iran "continues to threaten its neighbours and undermine regional security, including through its support for the Assad regime's murderous campaign in Syria, threats against the freedom of navigation in the region, and interference in Yemen," she charged.
The United States suspects Iran is sending arms to Assad's regime to help him crush a pro-democracy movement that UN officials estimate has cost more than 9,000 lives since it erupted in March 2011.
Iran has also threatened to close the Straits of Hormuz, a key world oil passageway, if the United States and European Union follow through on plans to impose an oil embargo over its controversial nuclear programme.
US Central Command chief General James Mattis has warned that Iran was sending support, including "weapons, not just money" to Huthi rebels in northern Yemen, and trying to "influence the non-Huthi tribes" as well.