Yemeni forces have surrounded a sprawling mosque complex in the capital Sanaa amid fears that backers of ousted autocrat Ali Abdullah Saleh might use it as a launchpad to attack the presidential palace.

The operation, in its fourth day on Tuesday, is the most dramatic standoff yet between current President Abd-Rabbu Mansour Hadi and Saleh’s supporters since he was forced to step down in 2011 following mass protests after 33 years in power.

Gulf neighbors and the West fear for the stability of Yemen, which shares a long border with top oil exporter Saudi Arabia. The Saudi government, along with Washington, helped push through the U.N.-mediated political transition. […]