BEIJING, March 18 -- The Chinese navy has shifted the focus of the search for flight MH370 westward and increased area under scrutiny six fold, an official said Tuesday.

Naval vessels on standby in the Gulf of Thailand headed north of the Singapore Strait on Monday evening and divided into two groups on Tuesday noon, said Liang Yang, a Navy spokesman.

The Jinggangshan amphibious docking vessel led a fleet through the Malacca Strait and into waters west of the Andaman Islands. Another fleet consisting of supply ship Qiandaohu, missile destroyer Haikou and amphibious transport dock Kunlunshan was heading for waters southwest of the Sumatra through the Sunda Strait, Liang said.

He noted the search faced difficulties. The depth of the sea exceeds 4,000 meters and the reflection from the seabed interferes with sonar.

The search for flight MH370, a Boeing 777-200ER, entered its 11th day on Tuesday. The plane went missing on its way to Beijing on March 8, carrying 227 passengers and 12 crew, including 154 Chinese.

Satellite data suggests the plane could be anywhere in either of two vast corridors: one stretching north from the Andaman Sea to the Caspian, the other south to the waters west of Australia, Liang said.

The navy would reinforce communication with authorities and keep adjusting the search, he said.