BANGKOK: An opposition leader in Thailand on Tuesday urged holdout parties to join an alliance with election winners to block defeated military-backed parties from forming a minority government.

The progressive Move Forward Party staked a claim as the new force in Thai politics on Sunday, winning most seats in a general election, including in some conservative strongholds, and coming within just four votes of a clean sweep of the capital, Bangkok.

Move Forward was closely followed by another opposition party, which before the vote had been tipped to win, the populist Pheu Thai, in a stunning rout of conservative parties that was widely interpreted as a rejection of nearly a decade of military-backed government.

The two parties agreed on Monday to form a ruling coalition of six parties, which together will have 310 of the 500 seats in the lower House of Representatives.

To ensure they can vote in a prime minister to form a government, however, the alliance needs not just a majority in the lower house, but more than half of lower and upper house seats combined, or 376 votes.

The big concern for the opposition is that it could be thwarted by the 250-member upper house Senate, which is appointed by the military, siding in a vote, as they tend to, with the conservative parties that lost on Sunday.

Those parties have yet to formally declare their intentions.

Srettha Thavasin, a real estate tycoon and senior Pheu Thai official, said parties weighing their options should honour the democratic pledges they made to the public and rally behind Move Forward's leader.

"It's time for you to follow through on your election pledge and vote for Move Forward's Pita Limjaroenrat according to the rules of a true democracy and not wait for the 250 senators," Srettha said in a statement.

"Political parties should follow the will of the people."

DECISIVE ROLE

The composition of Thailand's government could ultimately be determined by Bhumjaithai, a regional party that won 70 seats and has not announced its final decision.

Neither has the Democrat Party, which won 25 seats on Sunday - a small but potentially decisive number. Both are members of the current, military-backed administration.

Srettha reminded those parties they had both previously made clear they did not support unelected Senators deciding who forms a government.

"Now is the time for you to live up to your pledge and vote for Pita. You will have done your duty to your parties and the people with grace and dignity," he said.

Bhumjaithai, a party made famous by its a successful campaign to legalise the sale of marijuana, on Tuesday reiterated that its position was always that the winner should be allowed to form a government.

It said in a statement its executives would meet after the election result is certified, which could take up to 60 days, and would decide which direction to take, with the public's interest as the first priority.

Alongkorn Ponlaboot, a senior Democrat Party official, said there had been no party resolution on how its 25 newly elected lawmakers would vote, but the party "should respect the voice of the people by supporting Pita as prime minister".