Fresh sanctions slapped by the United States are meant to "expose and disrupt" links between Iran, Lebanon's armed Hezbollah movement and Syria, US Secretary of State Hillary Clinton said Saturday.

She said the "number one goal" of Washington and Ankara was to hasten the end of Bashar al-Assad's regime in Damascus and stop the bloodshed, while 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."

Washington on Friday announced sanctions against 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 it is under US 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 increase pressure on Damascus as the conflict escalates sharply after 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 was also due to discuss with Turkey's leaders 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.

Turkey, once a close ally of Syria, has become a vocal opponent of the regime since it 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 the neighbouring country 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 with Syrian activists and refugees, who she said were committed to "a free, inclusive and democratic Syria" but need support from the international 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 while predicting that more and more refugees would flee the country because of brutality which "knows no bounds" on the part of 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 currently home to some 55,000 refugees living in camps along the Syrian border, with close to 10,000 seeking safety this week alone amid intensifying battles between regime forces and rebels 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.