A German aid worker held for almost four months has escaped his kidnappers in Syria, just like his two colleagues who managed to flee in July, their aid group announced on Wednesday.

Engineer Ziad Nouri "is free" and managed to cross the Syrian border into Turkey on Tuesday, the Gruenhelme non-governmental organization said in a statement.

Prime Minister Recep Tayyip Erdogan said Wednesday that Turkey's ambassador to Egypt would return to Cairo, three weeks after he was recalled over the bloody crackdown on supporters of the country's ousted Islamist president.

"Do not forget that we recalled the ambassador for consultations," Erdogan told reporters at an Ankara airport before leaving for Saint Petersburg to attend the G20 conference.

An accidental explosion in a scrap metal warehouse killed six people in Syria just a few hundred meters from the Turkish border, media reported.

A Turk and five Syrians died in the mortar explosion on Tuesday caused by a shell going off without warning, television stations NTV and CNN-Turk reported.

Turkey's former army chief and more than 100 other suspects went on trial on Monday over a 1997 bloodless coup that toppled the country's first Islamist head of government.

General Ismail Hakki Karadayi stands accused with 102 co-defendants of "overthrowing the Turkish government by force."

Turkish police Sunday blocked the entrance to Istanbul's Gezi Park, the epicenter of anti-government protests in June, to prevent a demonstration there against a possible military intervention in Syria.

Riot police advanced with shields but held back from using tear gas or water cannon against about 1000 activists who instead formed a human chain on the city's celebrated Istiklal Avenue, according to an Agence France Presse photographer who witnessed the scene.

Turkey's prime minister said Friday that the limited strikes threatened by the United States against Syria were not enough and pushed for a broader military offensive that would topple the regime.

"A limited operation cannot be satisfactory for us," Recep Tayyip Erdogan was quoted as saying by the NTV news channel.

Turkey said on Friday that information collected by its secret services meant there was "no doubt" that the Syrian regime was behind a chemical attack on August 21.

Turkish Foreign Minister Ahmet Davutoglu told a press conference: "From our point of view, based on evidence gained by our intelligence services and experts... there is no doubt, it is clear the regime is responsible."

Any military response to alleged chemical weapons use by the Syrian regime of Bashar Assad will likely rest squarely on U.S. shoulders after Britain's refusal to commit its forces.

Once again at the spearhead of a possible lightning strike against Syrian military targets, the United States has reinforced its naval units in the eastern Mediterranean and could get help from France.

The kidnapped Turkish pilots urged their country on Thursday to "hurry up in releasing them," amid media reports that said they were being held near the southern suburbs of Beirut.

"We demand everyone to help in freeing the kidnapped Lebanese pilgrims,” pilot Murat Akpinar said in a recorded video message released on LBCI television.

The Turkish army has stepped up its "vigilance" following last week's suspected chemical weapons attack in Syria, Foreign Minister Ahmet Davutoglu said Wednesday.

"Since the chemical weapons (attack), we are of course in a situation of heightened vigilance," he told reporters while paying homage to Turkish soldiers patrolling the country's common border with Syria.