ISTANBUL — Twin car bombs killed at least 40 people near Turkey's border with Syria on Saturday, increasing fears that Syria's civil war was dragging in neighbors and drawing a swift warning from Ankara not to test its resolve.

Turkey supports the uprising against Syrian President Bashar al-Assad and Turkish Foreign Minister Ahmet Davutoglu said it was no coincidence the attacks in the town of Reyhanli came as diplomatic moves to end the conflict intensify. At least 100 people were injured in the attacks, the Associated Press reported.

"There may be those who want to sabotage Turkey's peace, but we will not allow that," Davutoglu told reporters during a trip to Berlin. "No-one should attempt to test Turkey's power, our security forces will take all necessary measures."

There was no immediate claim of responsibility.

NATO member Turkey has been one of Assad's harshest critics and has harbored both Syrian refugees and rebels during the uprising against him, now in its third year.

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Prospects appeared to improve this week for diplomacy over the civil war, in which more than 70,000 people have been killed, after Moscow and Washington announced a joint effort to bring government and rebels to an international conference.

But a Russian official said on Saturday that there was already disagreement over who would represent the opposition and he doubted whether a meeting could happen this month.

Smoke rose above Reyhanli, which lies in Turkey's southern Hatay province, after the blasts, which occurred 15 minutes apart and close to local administrative buildings.

Prime Minister Recep Tayyip Erdogan said this week that Turkey would support a U.S.-enforced no-fly zone in Syria and warned that Damascus crossed President Barack Obama's "red line" on chemical weapons use long ago.

A no-fly zone to prohibit Syrian military aircraft from hitting rebel targets has been mentioned by American lawmakers as one option the United States could use to pressure Assad.

Erdogan is due to meet Obama in Washington on May 16.

Violence has spilled over the border before. In February, a minibus blew up at a border crossing near Reyhanli, killing 14 people and wounding dozens more.

The Syrian opposition said one of its delegations appeared to have been the target of that attack, but there has been no confirmation of this from the Turkish authorities.

In October, five Turkish civilians were killed in Akcakale when a mortar bomb fired from Syria landed on their house, prompting Turkey to fire back across the frontier.

Turkey is sheltering more than 300,000 Syrians, most of them in camps along the 560-mile frontier, and is struggling to keep up with the influx.