Chinese Premier Wen Jiabao studies the disaster relief plan aboard a plane carrying him back to the quake-battered zone in southwest China's Sichuan Province on Thursday afternoon, May 22, 2008.

(Xinhua 2008-05-22 17:38:41)Chinese Premier Wen Jiabao made his second trip to the quake-battered zone on Thursday afternoon to supervise over disaster relief work.

Wen flew to Mianyang, in southwest Sichuan Province, one of the worst hit cities by the magnitude 8 earthquake on May 12.

The focus of relief work has shifted from rescuing lives to rehabilitating quake victims and local society, said the Premier on board of the plane on the way to Sichuan, "It will be a harder and long-term task."

The Ministry of Health has sent about 4,500 medical workers to implementsanitation work in quake-hit regions, Wen said.

At about 4:40 p.m., he oversaw a quake lake in Tangjiashan of Beichuan county on board of a helicopter.

In Sichuan, there are 33 "quake lakes" formed by landslides that blocked rivers. The one in Tangjiashan is one of the three largest.

Those lakes, formed as water was blocked from following river beds by rock and mud slides set off by the earthquakes, were very dangerous as the water level of some quake-lakes has risen, threatening vast areas.

His previous trip to the area was on May 12, just hours after the major tremor rocked Sichuan at 2:28 p.m.