John Kerry, the US secretary of state, is holding talks with Turkish leaders in Ankara over the crisis in Syria, amid reports of heavy fighting in Syria’s two main cities and a Scud missile attack at the Iraqi border.
In a joint press conference with Turkish Foreign Minister Ahmet Davutoglu on Friday, Kerry said the two discussed how the international community can come together "in an effort to try to create a political transition" in Syria.
"We need to continue to work to make certain that the [President Bashar al-Assad] regime makes a different set of choices," Kerry said.
"Minister Davutoglu and I, along with our partners, believe that there is no legitimacy in a regime that commits atrocities against its own people."
In-depth coverage of escalating violence across Syria
Kerry is also due to meet Turkish President Abdullah Gul and Prime Minister Recep Tayyip Erdogan.
The talks come a day after the US announced that it would for the first time provide direct aid to Syrian rebels in the form of food and medical supplies as well as $60m in extra assistance to the political opposition.
Russia has criticised the decision, saying the promised assistance would only intensify the nearly two-year-old conflict.
Kerry's meetings in Ankara are likely to be overshadowed by renewed tension between Turkey and Israel, two major US allies, following comments Erdogan made earlier this week, branding Zionism a "crime against humanity".
US officials said Kerry would raise the remarks directly with Erdogan.
Turkey, once a close ally of Syria, has joined the US in its campaign to overthrow the regime of Assad and has given shelter to Syrian rebels and to nearly 200,000 refugees along its volatile border.
But Turkey and the US differ on how to support Syria’s opposition.
"Kerry announced tens of millions of dollars in aid for the rebels but non-lethal aid, restricted to medical supplies and food for the rebels," Al Jazeera’s Nisreen El-Shamayleh, reporting from Antakya near the Syrian border, said.
"Turkey would like the US to do a little bit more than that. Turkey would like a buffer zone inside the northern part of Syria along with the border with turkey to protect people trying to flee the fighting."
In January, the US, along with Germany and the Netherlands, began deploying Patriot missiles as part of a NATO mission to protect Turkey from any spillover of the Syrian conflict.
Turkey had requested the measure after several cross-border shelling incidents, including one in October that killed five civilians.
Friday's diplomatic developments unfolded as Syrian government forces engaged in fierce clashes with rebels who attacked a police academy near the northern city of Aleppo.
The school, which activists say has been turned into a military base used to shell rebel-held neighbourhoods in the city and the surrounding countryside, has become a key front in the wider fight for Aleppo.
The Syrian state news agency said government troops defending the school had killed dozens of opposition fighters and destroyed five rebel vehicles.
Renewed clashes were also reported around Aleppo's landmark 12th century Umayyad Mosque in the walled Old City, which is a UNESCO World Heritage Site.
The mosque was heavily damaged in October 2012 just weeks after a fire gutted the old city's medieval market.
In another incident, a Scud missile fired from Syrian territory landed near a village in Iraq's Nineveh province on Friday, causing no damage but terrifying its inhabitants who fled, the mayor said.
In yet another incident, 10 bodies were discovered on a roadside between the Damascus suburbs of Adra and Dumair, the UK-based Syrian Observatory for Human Rights (SOHR) said.
All of the bodies were of men who appeared to be between the ages of 30 and 45, SOHR said. One of the men had been decapitated. Their identities were not immediately known.