UPDATE 4-Taliban kill 12 Pakistani soldiers in ambush

* Twelve soldiers killed, 10 wounded, in militant attack

* Army prepares assault on Pakistan Taliban chief

* U.S. General Petraeus visits

(Updates casualties from military statement)

By Hafiz Wazir

WANA, Pakistan, June 28 (Reuters) - Taliban militants ambushed a Pakistani military convoy near the Afghan border on Sunday killing 12 soldiers, the army said, as it prepared an assault on Pakistan's Taliban chief.

The military, near the end of an offensive in the northwestern Swat Valley after two months of fighting, is set to launch a new drive in South Waziristan, where Pakistani Taliban chief Baitullah Mehsud is based.

Pakistan's campaign against the Taliban has won the praise of close ally the United States, which needs Pakistan to go after the militants as it pursues its own strategy to defeat al Qaeda and stabilise Afghanistan.

The top U.S. commander for the region, General David Petraeus, arrived for talks with Pakistani leaders, the U.S. embassy said. Media said he met army chief General Ashfaq Kayani.