Most Southeast Asian stock markets opened lower on Monday as fears mounted of an escalation in Sino-U.S. trade tensions after news that Washington is considering de-listing Chinese companies from U.S. stock exchanges.

The U.S. administration is considering de-listing Chinese companies to limit U.S. investment in Chinese companies, Reuters reported on Friday quoting sources briefed on the matter.

"The timing of this story right on the cusp of another round of high-level trade talk between the world's most massive economies adds another layer of muck to an already befouled trade war narrative," said Stephen Innes, Asia Pacific market strategist at AxiTrader.

Adding to the gloom, China's factory activity shrank for the fifth straight month in September, pointing to persistent pressure on Southeast Asia's biggest trading partner from the protracted trade war.

Indonesian shares fell the most in Southeast Asia, weighed down by financial and consumer stocks.

Consumer goods company Unilever Indonesia lost 1.3%, while lender Bank Central Asia was down 0.7%.

An index of Indonesia's 45 most liquid stocks was 0.7% lower.

For the month, Indonesian shares were down over 2%, heading for a second straight monthly drop.

Singapore shares fell, dragged by industrial and telecommunication stocks, but were on track to post their first monthly gain in three.

Thai shares edged lower ahead of August trade balance data. Exports rose 3.8% on year in July, but current account surplus narrowed from a month earlier.

Finance and consumer stocks led the losses. Lender Kasikornbank PCL slipped 1.6%, while convenience stores operator CP All PCL lost 0.6%.

Thai shares were down 0.7% for the month, on track for a third monthly drop.

Vietnam shares edged higher, helped by the real estate sector. Vingroup JSC rose 0.9%, while Vinhomes JSC rose 0.6% each.

Vietnam's economic growth quickened to 7.31% in the third quarter year-on-year, the General Statistics Office said on Saturday.

The Asian Development Bank had said earlier that it maintained its forecast for Vietnam's growth for this year at 6.8%, and for next year at 6.7%.