Boeing Co. is contending with a growing crisis after Singapore and Australia closed their air space to 737 Max planes in response to a second crash of the model in five months.

While investigators are still piecing together why an Ethiopian Airlines 737 Max 8 plunged to the ground minutes after takeoff Sunday, the two nations opted for caution, following the lead of China and Indonesia—even as the U.S. Federal Aviation Administration has said the jet model remains airworthy.

Boeing shares slipped further in pre-market U.S. trading Tuesday, after tumbling the most on the S&P 500 Index the day before amid swirling questions around the newest version of its 737 family, a cash cow that generates almost a third of the company’s operating profit.

Indonesia’s Lion Air, one of the biggest customers for the 737 Max, is refusing to take delivery of more planes and is considering switching to Airbus SE, escalating a tense standoff that resulted from the carrier’s own air disaster which killed 189 people in October, according a person familiar with the matter.

Anger over the U.S. planemaker’s response to the Lion Air tragedy prompted co-founder Rusdi Kirana to say in December he planned to scrap $22 billion of Boeing jet orders. The Ethiopian crash, which bore some similarities to Lion Air’s, has added to Kirana’s resolve, the person said.

Eastar, Silkair

South Korean operator Eastar Jet said it will suspend flights with its two 737 Max 8 planes, while Singapore’s Silkair said temporarily withdrawing its fleet of six Max 8’s will affect some of its flight schedules. These followed Max jet groundings by low-cost Brazilian carrier Gol Linhas Aereas Inteligentes SA, along with Grupo Aeromexico.

The Civil Aviation Authority of Singapore said all Boeing 737 Max flights will be suspended into and out of Singapore. The suspension takes effect from 2 p.m. local time on March 12.

Mexico’s Grupo Aeromexico SAB became the latest airline to suspend Boeing 737 Max flights. The airline said it would temporarily ground its six Max jets until it has more information about the Ethiopian Airlines disaster.

Hours earlier, Aeromexico said it was confident in its Max fleet and the aircraft were in perfect condition.