Fresh US sanctions aim to "expose and disrupt" links between Iran, Lebanon's armed Hezbollah movement and Syria, Secretary of State Hillary Clinton said Saturday.
The "number one goal" of Washington and Ankara was to hasten the end of Bashar al-Assad's regime and stop the bloodshed, she said, also warning that Syria must not become a haven for Kurdish rebels battling Turkey.
"We are continuing to increase pressure from outside," Clinton told a joint press conference in Istanbul after meeting Turkish Foreign Minister Ahmet Davutoglu and Syrian opposition activists.
"Yesterday in Washington we announced sanctions designed to expose and disrupt the links between Iran, Hezbollah and Syria that prolong the life of the Assad regime," said Clinton, who later left to return to Washington.
The sanctions target Syrian state oil company Sytrol for trading with Iran, in a bid to starve both Tehran and Damascus of much-needed revenue.
The US Treasury also said it was adding the Lebanese Shiite militant group Hezbollah, which has close ties with Iran and Syria, to a blacklist of organisations targeted under Syria-related sanctions.
Washington already classes Hezbollah a "terrorist organisation" and has hit it with sanctions, but Friday's move explicitly ties the group to the violence in Syria, where Assad is attempting to put down a 17-month revolt.
The sanctions are designed to heighten pressure on Damascus as the conflict is escalating sharply following the failure of former UN-Arab League envoy Kofi Annan's peace plan and his dramatic resignation.
World powers are expected to name veteran Algerian diplomat Lakhdar Brahimi as their new envoy for Syria early next week.
Clinton also met Prime Minister Recep Tayyip Erdogan and President Abdullah Gul to discuss ways to effectively enforce sanctions against Syria and accelerate efforts for the fall of the Assad regime.
"Our goal number one is to hasten the end of the bloodshed and the Assad regime, that is our strategic goal," she said.
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Turkey, once a close ally of Syria, has become a vocal opponent of the regime which launched a brutal crackdown on dissent in March last year.
It has imposed its own sanctions, taken in tens of thousands of refugees and allowed the rebel Free Syrian Army to operate from its soil.
Praising Turkey's leadership in the Syrian conflict, Clinton also said she shared Ankara's determination to prevent the outlawed Kurdistan Workers Party (PKK) from using neighbouring Syria as a base.
"We share Turkey's determination that Syria must not become a haven for PKK terorrists whether now or after the departure of the Assad regime," Clinton said.
The Democratic Union Party (PYD), the Syrian ally of the PKK, has reportedly taken over several towns along Turkey's border with Syria, alarming Ankara, which promptly increased defences on the border.
"We need to take joint efforts to prevent a power vacuum from being formed" which could be exploited by the PKK, Davutoglu said.
Clinton also met Syrian activists and refugees who she said were committed to "a free, inclusive and democratic Syria" but would need support from the world community to be ready for the eventual fall of the Assad regime.
"We share the frustration, anger and outrage of the Syrian people," she said, predicting more and more refugees because of the brutality which "knows no bounds" on the part of Syrian government forces.
She announced an additional $5.5 million in aid for those fleeing fighting that monitoring groups say has now claimed over 21,000 lives.
Turkey is now shelters some 55,000 refugees in camps along the Syrian border, with close to 10,000 seeking safety this week alone amid intensifying battles for Syria's second city of Aleppo.
Clinton flew into Istanbul early Saturday from Benin after wrapping up a marathon 11-day, nine-nation Africa tour.