BAGHDAD, April 1 (Reuters) - About half of the nearly 30,000 additional U.S. troops being sent to Iraq in a new effort to stabilise the country have now been deployed, a senior U.S. military official said on Sunday.

The rest were either in staging areas in neighbouring Kuwait or on their way, Rear Admiral Mark Fox told a news conference in Baghdad. In total there are about 140,000 U.S. troops now in Iraq.

"We expect the remainder to be in place by early June," he said.

Most of the troops are being sent to Baghdad for a U.S.-backed security crackdown to curb rampant sectarian violence. Reinforcements are also being sent to western Anbar province, heartland of the Sunni Arab insurgency.

Fox cautioned that the success of the Baghdad crackdown, codenamed Operation Imposing Law, would not be seen in days or weeks but in months. He was speaking after the bloodiest week since the operation began in mid-February.

Suicide bombers struck across the country, mainly outside Baghdad, killing more than 400 people in attacks blamed on Sunni Islamist al Qaeda.

"Al Qaeda continues to carry our brutal attacks to disrupt our efforts ... and to instigate sectarian violence," Fox said.

He said there had been a serious security breach this week in the heavily fortified Green Zone in central Baghdad, which houses the Iraqi government and the U.S. embassy.

Two explosives vests of the kind typically worn by suicide bombers had been found, he said, without giving the exact location of the discovery. An investigation was under way.

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