The Thai benchmark led declines in the region and was down 1.6% - its lowest closing level in three weeks.
Financials and industrials were among the top drags on the index, with Kasikornbank Pcl and Airports of Thailand losing 2.1% and 1%, respectively.
Iran fired more than a dozen missiles against at least two Iraqi facilities hosting U.S.-led coalition personnel. However, Iranian officials said Tehran did not want a war and its strikes "concluded" its response to Friday's killing of its top general by the United States.
U.S. President Donald Trump said an assessment of casualties and damage from the strikes was under way and tweeted "all is well".
Singapore's benchmark index recovered most of its early losses to end marginally lower. It had fallen as much as 1.7% earlier in the session.
Most investors are not expecting the situation to worsen, given that both the sides are facing pressure not to escalate matters further, said Joel Ng, an analyst at KGI Securities.
"There are no reports yet of casualties on the American side, and with President Trump tweeting "all is well", it seems that it will be calmer now."
Heavyweights DBS Group Holdings and Jardine Matheson Holdings lost 1.2% and 0.3%, respectively.
The Malaysian index ended at over a week low, weighed down by financial and consumer sectors.
Public Bank slipped 2.6%, while Sime Darby Plantation lost 0.9%.
Philippine bourse slipped to a three-week low, dragged by losses in blue-chip stocks, with BDO Unibank and industrial conglomerate JG Summit Holdings falling 1.9% and 2.8%.
Indonesian shares ended at over a three-week low, with Bank Mandiri (Persero) losing 1.3%, while state-owned telecom firm Telekomunikasi Indonesia ended 1% lower.
An index of Jakarta's 45 most liquid stocks <.JKLQ45> slipped 0.9%.
Vietnam stocks ended at their lowest in over six months, with Vinhomes JSC losing the most in real estate sector.
By Sameer Manekar