Turkish security forces seized explosives and four suicide vests in the baggage of a group of suspects stopped at the border with Syria, the Turkish army said on Wednesday.

"Between 12 and 15 kilos of explosives and four belts that could be used for suicide attacks were found in two bags," the army said in a statement.

A group of 34 people - four men, 10 women and 20 children - were stopped in Turkey's southeastern border town of Karkamis, the statement said.

The army did not refer to any terror group in its statement, but Karkamis lies across the border from the Syrian town of Jerabalus, which is currently controlled by the Islamic State group.

The seizure comes as around 30,000 Syrians are huddled on the border with Turkey at Oncupinar, around 100 kilometres west of Karkamis, after fleeing a major government offensive, backed by Russian air power, in Aleppo.

Turkey, already hosting 2.7mn Syrian refugees, has so far refused to let the new wave into the country, providing humanitarian assistance across the border.

The UN on Tuesday urged Turkey to open its borders to the Syrians, who have overwhelmed the emergency camps, where huge crowds including women and children have been forced to to sleep in tents or in the open.

Turkey's Deputy Prime Minister Numan Kurtulmus has said that a "worst case scenario" could see up to 600,000 refugees arrive at the border.

Turkish police are on the alert for fighters slipping into the country, after a series of attacks on its soil claimed by the IS group.

In January 11 German tourists were killed in a blast in Istanbul's historic centre, while in October last year a suicide attack killed 103 people at a demonstration in Ankara.