A Syrian fighter jet bombed the town of Ras al-Ain for a second day, close to Turkish border.
Forces loyal to President Bashar al-Assad are trying to regain control of Ras al-Ain, which fell to rebels last Thursday.
The offensive has caused some of the biggest refugee movements since the Syrian conflict began in March last year.
In one 24-hour period last week, some 9,000 Syrians fled fighting during a rebel advance into Syria's northeast.
Rebels fired machineguns mounted on the back of pick-up trucks into the air as the jet swooped low over Ras al-Ain, dropping three bombs before returning for a second strike on another part of the town.
Turkey has repeatedly fired back in retaliation for stray gunfire and mortar rounds flying across its 900km border with Syria, and is talking to NATO allies about the possible deployment of Patriot surface-to-air missiles.
The number of registered refugees in Turkish camps has swelled to over 120,000. Tens of thousands more are unregistered and living in Turkish homes.
Syrian jets and artillery hit the town of Albu Kamal on the frontier with Iraq, where rebels have seized some areas, according to the mayor of the Iraqi border town of Qaim.
Tension also remained high on the Golan Heights, where Israeli gunners have retaliated against stray Syrian mortar fire landing on the occupied plateau in the previous two days.
The Syrian Arab Red Crescent estimates that 2.5 million people are internally displaced within Syria by civil war.
The United Nations refugee agency said "the figure they are using is 2.5 million. If anything, they believe it could be more, this is a very conservative estimate," Melissa Fleming, chief spokeswoman of the UNHCR said.
"So people are moving, really on the run, hiding. They are difficult to count and access," she said.
Meanwhile, France's Defence Minister said it was still too early to recognise the newly created Syrian opposition coalition, calling for more to be done to unite the armed factions under its umbrella.
Leaders of Syria's exiled opposition formed the coalition in Qatar's capital Doha on Sunday and the grouping is now seeking international recognition as a government-in-waiting.
Paris, one of President Assad's harshest critics, has said it would recognise a provisional government that included all strands of society.
But it has ruled out arming rebel forces, concerned weapons could get into the hands of radical Islamists.
British Foreign Secretary William Hague said the coalition was an important step forward but more needed to be done before Britain formally recognised it.
"It is a very important milestone," Mr Hague told reporters at a meeting of Arab and European ministers at the headquarters of the Arab League in Cairo.
"We want to see the Syrian opposition be inclusive ... and have support inside Syria and if they have this, yes, we will then recognise them as the legitimate representative of the Syrian people".