ISTANBUL — For the second time in a week, the civil war in Syria spilled across border areas on Wednesday as rebel forces were reported to have driven government troops from a northern frontier crossing in an apparent effort to expand supply and infiltration routes in the campaign to overthrow President Bashar al-Assad.

Turkish schools in the region were closed for the day after intense overnight clashes in which the rebels attacked the Syrian frontier post at Tal Abyad, south of the Turkish town of Sanliurfa, according to the semiofficial Anatolian News Agency. Television images appeared to show members of the insurgent Free Syrian Army standing on the roof of the border post and hauling down the Syrian flag at Tal Abyad, which is less than a mile from Turkey’s Akcakale crossing.

A private Turkish television channel said Syrian tanks were headed for the border post. The attack came two days after Lebanese officials said that Syrian warplanes and helicopters, pursuing opposition fighters, had fired on a Lebanese town near the two countries’ shared border.