A statement issued Saturday by a representative for Catherine Ashton, the European Union's High Representative for Foreign Affairs and Security Policy, indicated alarm at the move, which was made Wednesday by the Lebanese government.
Some of the deportees from Lebanon were activists opposed to the regime of embattled Syrian President Bashar Assad and could face torture or death upon return to Damascus, the Lebanese newspaper The Daily Star reported.
"The High Representative is concerned by the recent reports of expulsion by the Lebanese authorities of Syrian nationals to Syria," the statement said. "As a party to the U.N. Convention Against Torture, Lebanon has a clear legal obligation that no person is returned or extradited to a state where they could be subjected to torture."
Lebanese security officials asserted the deported Syrians were criminals who had been sentenced for crimes, such as theft, forgery, attempted rape and assault, committed on Lebanese territory.
Former Lebanese Prime Minister Fouad Siniora and other opposition leaders contended the expulsions were ordered by Assad's regime, the newspaper said.
The opposition March 14 parliamentary coalition Friday demanded the resignation of Prime Minister Najib Mikati's government over the deportations.
The EU statement warned "Syria is in a situation where people's lives are at risk across the country, including well documented reports of torture," and demanded that Lebanon "must ensure that no deportation takes place outside the framework of its international obligations" agreed upon with the U.N. High Commissioner for Refugees.
Ashton's office acknowledged Lebanon is playing an important role in coping with the thousands of refugees fleeing the fierce fighting in the Syria, as heavily armed rebels seek to topple Assad's government.
The UNHCR reported Sunday more than 35,000 Syrian refugees in Lebanon were receiving assistance with the help of the government, with more pouring in every day.
The agency said a steady stream of arrivals from the Syrian cities of Homs, Damascus, Dara'a and Aleppo are making for Lebanese destinations such as Beirut, Saida and Tripoli to live with family and friends or to rent apartments.
Of the more than 33,000 registered and 1,700 awaiting registration in Lebanon, many are living in poor communities in North Lebanon and Bekaa Valley.
But the European Union also said it expects Lebanon "to continue to provide protection, in line with the principle of non-refoulement" -- which forbids handing over a victim of persecution to their persecutor.
Other Western countries have expressed alarm over the move deport the Syrian refugees, which they fear could set a negative precedent.
U.S. Ambassador to Lebanon Maura Connelly said Thursday Washington was "deeply disturbed" by the actions, adding the United States "stressed the importance of protecting all Syrians, including dissenters and deserters, who have rejected violence, in keeping with Lebanon's international humanitarian obligations."
France also decried the expulsions, the Kuwait News Agency reported.
"The escalation in the violence in Syria as well as the threats of torture or death which justified the fleeing to Lebanon by certain Syrian nationals makes it necessary to provide effective protection to these persons," the French Foreign Ministry said in a statement.
International humanitarian law, Paris said, demands the "protection and non-expulsion of these people must be maintained."