CAIRO (Reuters) - Egypt has turned back several flights carrying hundreds of Syrians after tightening visa rules following the toppling of President Mohamed Mursi, the U.N. refugee agency said on Friday.

Egypt this week introduced visa requirements for Syrians after local media accused Syrian Islamists of joining deadly clashes between Mursi's supporters and the army who had ousted him. They previously only needed a passport to enter Egypt.

Hundreds of thousands of Syrians have fled a conflict pitting forces loyal to President Bashar al-Assad against insurgents, which has killed an estimated 100,000 people and driven 1.7 million abroad.

UNHCR spokeswoman Melissa Fleming said Egyptian authorities had refused permission for several flights from Damascus and the Syrian coastal city Latakia - one with 250 Syrians on board - to land. Syrians landing on flights from other countries were deported upon arrival at Cairo airport.

"I appeal to the Egyptian authorities ... admit and protect all Syrians seeking refuge in their country," U.N. High Commissioner for Refugees Antonio Guterres said. The Egyptian embassy in Damascus was not able to issue visas at the moment, the UNHCR said.

Monday's clash between Mursi supporters and soldiers in Cairo, in which 53 protesters and four soldiers were killed, has stoked fears among some Egyptians that the trouble is being stirred up by outsiders.

They are especially wary of those from Syria and Gaza, home to Islamist groups seen as close to Mursi's Muslim Brotherhood.

(Reporting by Ulf Laessing; Editing by Mike Collett-White and Alison Williams)