US Secretary of State Hillary Clinton arrived Saturday in Istanbul for talks on the conflict in Syria with Turkish leaders after Washington slapped fresh sanctions on Bashar al-Assad's regime and its allies.
Clinton flew in after wrapping up a nine-nation Africa tour.
The visit comes after Washington on Friday announced sanctions on Syrian state oil company Sytrol for trading with Iran, in a bid to starve the regimes in 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 the Assad regime as the conflict escalates sharply after the failure of former UN-Arab League envoy Kofi Annan's peace plan and his dramatic resignation.
In Istanbul, Clinton will have "lengthy and in-depth conversation" with Turkey's president, prime minister and foreign minister to discuss a three-pronged strategy, a US official said.
The first aspect is "how we judge the effectiveness of what we are doing in terms of supporting the opposition," the official said, adding that pressure and isolation of the regime was part of the strategy.
Clinton is expected to discuss with Turkey's leaders ways to effectively enforce sanctions against Damascus.
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.
Relations hit an all-time low after a Turkish fighter jet was shot down by Syrian fire in June, killing its two-man crew and leading Ankara to brand Damascus a "hostile" opponent.
In November, the Turkish government joined Arab League sanctions, freezing Syrian government financial assets, imposing a travel ban on senior Syrian officials and cutting off transactions with the country's central bank.
The second part of the strategy, according to the US official, would be to extend humanitarian assistance to Turkey as it copes with an influx of refugees from Syria.
In Istanbul, Clinton is expected to announce 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 more than 50,000 refugees living in camps along the Syrian border.
This week has seen a marked increased in refugees, with close to 10,000 turning up at the border amid intensifying battles between regime forces and rebels for Syria's second city of Aleppo.
The third strategy, the US official said, is built on a transition plan out of the "strong conviction" that Assad's days are numbered and that the international community needs to be prepared to support Syrians.
US President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan agreed at the end of July to work on political change in Syria, including Assad's departure.
Turkey is also providing sanctuary to forces defecting from Assad's army to link up with the opposition Free Syrian Army, some of whose leaders are based on Turkish soil near the border.