South Korea's independent presidential candidate Ahn Cheol-soo expressed disappointment Thursday as talks with his liberal rival over merging their campaigns hit a snag.

Ahn and Moon Jae-in, the presidential nominee for the center- left main opposition Democratic United Party, have been briefly engaged in talks over fielding a single candidate against their conservative archrival, Park Geun-hye of the conservative ruling Saenuri Party.

The Ahn campaign, however, unilaterally called off high-profile talks a day ago, accusing the Moon campaign of spreading rumors that Ahn will eventually concede to Moon.

"I am deeply disappointed," said Ahn, a 50-year-old political novice who founded a antivirus software firm. "If the process of selecting a single candidate is viewed as competition, the one who wins will not be able to win the election."

Moon, a 59-year-old former human rights lawyer who served as chief of staff to late President Roh Moo-hyun, has apologized for upsetting Ahn and his aides and asked the independent candidate to return to the negotiating table.

"I will take all the necessary measures" to end the controversy, Moon told reporters. "I apologize if the campaign or the Democratic United Party said or did anything inappropriate."

Prospects for a possible alliance between the two men have been seen as a game changer that would determine the fate of the December presidential election, as they are seen as liberal alternatives to Park.

A handful of key figures of the Democratic United Party, including Moon loyalists, have stepped down as the Ahn campaign consistently called demands for political reform on the part of the main opposition party.

Before the merger talks came to an abrupt halt, Moon and Ahn were expected to soon unveil a joint declaration outlining their plan for political reform. The two men were also planning to take part in televised debates.

The presidential election is slated for Dec. 19, with President Lee Myung-bak constitutionally barred from seeking re-election.