Thai Prime Minister Abhisit Vejjajiva admitted Monday that the country's northeast is still a stronghold for the opposition Puea Thai party after it won by-elections in two provinces, local media reported.

Candidates from the Puea Thai party have defeated its rivals in northeastern provinces of Sakon Nakhon and Si Sa Ket in recent parliament by-elections.

The reason that he decided not to visit the northeastern provinces during the by-election campaigns is because he did not want to be the cause of conflicts, not because he was scared of the Puea Thai supporters, the Thai-language INN news agency quoted Abhisit as saying.

When asked former Prime Minister Thaksin Shinawatra's phone-in speeches to people in the two northeastern provinces could be the key to the opposition's back-to-back wins, Abhisit said the reporters should asked the voters themselves.

Referring to the reports of a campaign to gather one million signatures of people to support a petition to His Majesty the King to seek a royal pardon for Thaksin, the prime minister insisted the problems of Thaksin should stay within the legal and political process.

Supporters led by the anti-government United front for Democracy against Dictatorship (UDD), staged a mass rally at Bangkok's  Sanam Luang to show their support for Thaksin. Core leaders of the UDD vowed to seek royal pardon for the ousted prime minister.

Thaksin was ousted by a military coup in September 2006, in accusation of corruption, and kept in exile since then. He returned to Thailand in February 2008 to face corruption charges, but later fled into exile again and was convicted in absentia.