* Insurgents gain control of third border gate with Turkey

* Region in northern Syria had been solidly pro-Assad

* Two Turkish civilians wounded inside Turkey (Recasts with rebels seizing crossing, adds details)

By Kadir Celikcan

AKCAKALE, Turkey, Sept 19 (Reuters) - Syrian rebels seized their third border crossing with Turkey on Wednesday, a Turkish official said, after fierce battles with government troops overnight that sent bullets flying into Syria's northern neighbour.

Television footage showed a rebel tearing down the Syrian flag on top of what appeared to be a customs building at the Tel Abyad frontier gate. Minutes earlier sporadic gunfire could be heard and black smoke rose from parts of the building.

"I can confirm that the gate has fallen. It is under the complete control of the rebels," a Turkish official told Reuters

on condition of anonymity.

There was no sign of any government troops at the crossing from television pictures broadcast live on CNN Turk.

The clashes, which started late on Tuesday, were the first time insurgents fighting to overthrow Syrian President Bashar al-Assad have asserted their grip over a border zone in Syria's al-Raqqa province, most of which has remained solidly pro-Assad.

Rebels hold two other crossings on the northern border with Turkey. A third border point will strengthen their control in the north and put more pressure on Assad's army as the two sides battle for control of Syria's largest city Aleppo not far away.

The governor's office in the small town of Akcakale, on the Turkish side of the border post, ordered all schools in the town and the neighbouring villages to close for the day and banned all agricultural work in the area.

"A heavy hail of bullets is landing here. We are scared. We had to stay in another house last night. We don't know what to do," a man in his forties told CNN Turk hours before the post was seized.

"Teachers, everyone have left the school next to us, they have fled the area," he said, standing only metres (yards) from the fence separating the countries.

Some 300 Syrians had fled over to Turkey around Akcakale to escape the fighting, the Turkish official said. Twenty-five rebels wounded during the clashes were also receiving medical treatment in Turkey.

Syrian jets bombed the Syrian town of Abu Kamal near the Iraqi border on Wednesday, Iraqi security officials and the mayor of Iraqi frontier town al-Qaim said. The two towns are metres away from each other on the banks of the River Euphrates.

TWO WOUNDED

A Turkish woman and her daughter were wounded on Tuesday night by stray bullets and an official said other bullets had smashed windows in several houses in Turkey along the border.

Ankara has yet to give a reaction to the fighting along its frontier but a similar incident earlier this year prompted a sharp rebuke from the government.

Turkey officially reported to the United Nations an incident in April in which at least five people, including two Turkish officials, were wounded when cross-border gunfire hit a Syrian refugee camp in Kilis further west along the frontier.

Turkish Prime Minister Tayyip Erdogan then floated the idea of invoking NATO's Article 5 over the incident, saying the alliance had a duty to protect its members' borders.

Article 5 of the NATO treaty states that an armed attack against one of its members will be considered an attack against all members and allows for the use of armed force.

Erdogan spoke to U.S. President Barack Obama late on Tuesday, his office said in a statement. The two leaders discussed the crisis in Syria among other issues. The statement made no reference to the border incident.

Once an ally of Assad, Erdogan is now among his most vocal critics and has called for him to step down. Turkey actively supports the anti-Assad rebellion, giving fighters sanctuary on its soil and allowing opposition members to meet in Turkish cities.

It is also sheltering some 83,000 Syrians who have fled the violence in camps along the border.

The 18-month-old revolt, which began as peaceful street protests cracked down on by Assad's military, has escalated into a civil war in which over 27,000 people have died. Daily death tolls now approach 200 and the last month was the bloodiest yet. (Additional reporting and writing by Jonathon Burch in Ankara and Suadad al-Salhy in Baghdad; Editing by Nick Tattersall and Mark Heinrich)