BANGKOK (Reuters) - Southeast Asian stock markets mostly slipped on Thursday, with Indonesia losing the most as investors cut risk exposures after the US Federal Reserve's policy outlook was seen as less dovish than expected.

Jakarta's Composite Index dropped 1.7 per cent to 4,497.12, trimming its gain so far this month to 4.2 per cent, after a 2.9 per cent rise a month earlier.

Large caps underperformed the broader market, with the blue chip index down 2.1 per cent. Selling was most active in Bank Rakyat Indonesia, Bank Mandiri and Perusahaan Gas Negara.

In Bangkok, the benchmark SET index edged down 0.02 per cent after a 1.7 per cent drop on Wednesday due to political concerns. It is set to extend its gain for a second month, up 3.5 per cent.

Short covering helped reverse losses in shares of large caps including Advanced Info Service. Industrial conglomerate Siam Cement lost 1 per cent after its downbeat 2013 revenue forecast.

"Rising political tension over the amnesty bill will likely remain the key factor to dampen market sentiment today... Although the SET may experience an intraday technical rebound, we would trade cautiously," broker Krungsri Securities said in a strategy note.

The Thai lower house debated a political amnesty bill on Thursday which is seen as aiming to annul legal cases involving a coup in 2006, while anti-government protesters vow to begin a rally later in the day.

In Singapore, weak quarterly earnings triggered selling in shares, with property developer CapitaLand losing 1 per cent, underperforming the Straits Times Index which fell 0.2 per cent.

Philippine shares bucked the trend, with the main index edging up 0.07 per cent after falling almost 1 per cent on Thursday morning. Trading volume was 1.2 times the full-day average over the past 30 sessions.

Southeast Asian shares are on track to post gains for a second successive month, with Singapore poised for a 1.8 per cent rise, Malaysia set to add 2.3 per cent, the Philippines 6.6 per cent and Vietnam 1.3 per cent.