"Chief of Staff Im must tell the truth before the National Assembly before it escalates further," Chang Je-won, the spokesman of the main opposition Liberty Korea Party, told reporters, calling the case "nuclear reactor gate."

Chang also touched on the news reports that Suh Dong-gu, the first deputy director of the National Intelligence Service (NIS), accompanied Im during his visit to the UAE.

"Why did he have to bring the first NIS deputy director in charge of overseas intelligence there when he said his trip was intended to encourage South Korean troops there," he said. "Is the deputy director his (personal) secretary?"

The minor opposition People's Party stressed the "responsibility" for the presidential office to address mounting speculation over Im's trip.

"Despite rumors being circulated, Cheong Wa Dae has failed to properly explain the concrete purpose of his trip and the outcome of discussions (with top UAE officials)," Kim Dong-cheol, the party's floor leader, said during a party meeting.

"It is not a matter for the ruling Democratic Party to cover up or just dismiss as a political offensive," he added.

Yoo Seong-min, the leader of the Bareun Party, even said that the legislature may have to seek a parliamentary probe into the case.

"If there is any friction or serious problem between the two countries regarding the construction of reactors in the UAE, the parliament must verify them," he said during a party meeting.

In the UAE, South Korea has the "Akh" unit of more than 140 troops that assists in training UAE soldiers. "Akh" means "brothers" in Arabic. (Yonhap)