Most Southeast Asian stock markets edged lower on Monday, in line with broader Asia, as caution gripped investors ahead of the U.S. Federal Reserve's two-day policy meeting that starts on Tuesday.

With a 25 basis point rate hike seen as a done deal, a key focus is on whether Fed policymakers forecast four rate hikes this year in their "dot plot" projections, instead of three they projected at a December meeting.

"As far as equities go, I think people will be a bit more risk-off if there are more rate hikes than expected," said Fio Dejesus, an equity research analyst with RCBC Securities.

Thai shares were the biggest losers in Southeast Asia, hurt by weakness in energy stocks. Oil and gas firm PTT Pcl dropped 2.2 percent.   

Singapore shares fell for a fourth straight session, with financials among the top losers. Lender Oversea-Chinese Banking Corp Ltd declined 0.9 percent, while DBS Group Holdings Ltd slipped 0.6 percent.

Indonesian shares slipped, weighed down by consumer discretionary and materials stocks.

Astra International Tbk PT fell 2.4 percent to its lowest close July 2016.

An index of the country's 45 most liquid stocks slipped 0.1 percent.

Philippine shares recovered from early falls to close marginally lower with financial and consumer discretionary stocks leading the recovery. Ayala Corp ended 2 percent higher.

"You have to take into account that the Philippines has already been correcting for the past two weeks, so perhaps the valuations are now better than they have been recently," said Dejesus.

The benchmark stock index has declined 3.8 percent year-to-date after gaining about 25 percent in 2017.

Malaysian shares closed slightly higher, helped by gains in financials and telecom services stocks. Public Bank Bhd gained 0.4 percent.

Vietnam shares rose for a sixth straight session, helped by real estate stocks and industrials, and posted their highest close since March 2007.