Human Rights Watch (HRW) and four other groups alleged in a report on Friday that staff at Lebanon's General Security Directorate deported at least 16 Syrians after forcing them to sign "voluntarily repatriation forms".
At least five of the 16 were registered refugees, at least 13 "expressed their fears of torture or persecution if returned to Syria", the rights groups said in a statement.
Lebanon's General Security Directorate said it "categorically denies it forced any Syrian to sign any form", in a statement carried by the official NNA news agency.
"Any Syrian who arrives in Lebanon and does not meet entry requirements and ... wants to go to Syria because they do not wish to remain in their country of residence for a number of reasons, signs a declaration of responsibility for choosing to return voluntarily," it said.
The statement, however, noted that the agency "has been working and coordinating with all international organisations" to facilitate refugee returns.
The latest deportees said they were "pressured" by the government agency at the airport, HRW said.
"Lebanese authorities shouldn't deport anyone to Syria without first allowing them a fair opportunity to argue their case for protection and ensuring that they don't face a real risk of persecution, torture, or other serious harm," said Lama Fakih, HRW's acting Middle East director.
One of the deportees said that the United Nations refugee agency the UNHCR was not notified that he was being deported, and when it did hear of the expulsion, was not able to stop it.
Lebanon has hosted more than 1.5 million Syrian refugees, a significant burden for a country of some 4.5 million people since the Syrian civil war's outbreak in 2011.
While fighting has slowed or ended in many areas of Syria, the UN has stressed that all returns should be voluntary.
The rights groups say some 74 percent of Syrians in Lebanon lack legal residency and are at risk of detention.
Local media in Lebanon have reported that the Supreme Defence Council, whose decisions are not made public, recently instructed General Security Directorate to deport all Syrians who have entered the country illegally.
The official NNA news agency, quoting a "security report", said on Friday that Lebanese authorities had deported 301 Syrians between May 7 and May 20.
Syria's war has killed more than 370,000 people and displaced millions inside the country and abroad.
The war was triggered in March 2011 by a violent crackdown on anti-government demonstrations.