U.S. lawmakers condemned a brutal offensive by Myanmar’s army against ethnic minority civilians, calling on the U.N. Security Council to take urgent action against the country’s ruling junta, statements received Thursday said.

Myanmar troops have uprooted more than 11,000 Karen civilians in their biggest offensive in almost a decade a campaign punctuated by torture and killings, according to reports from inside the country and Thailand.

Troops in eastern Myanmar have torched Karen villages and sent inhabitants fleeing into jungles in a campaign to suppress a decades-old insurgency, say reports from Free Burma Rangers, a group of Westerners and ethnic volunteers who provide aid to displaced people in the country, formerly known as Burma.

Tom Lantos, the top Democrat on the U.S. House International Relations Committee, said the attacks were a “deadly escalation of what is already one of the world’s most serious humanitarian disasters.”

“The Burmese military junta must call off its campaign against innocent civilians,” he said

He called on the U.N. Security Council to follow up on its debate last December on Myanmar with a binding resolution aimed at stopping the violence. “The longer the Security Council waits, the more villages will be destroyed and more people will die,” he said.

Some 11,000 people have fled their homes due to the onslaught, which began last November and has recently intensified, the Free Burma Rangers said. About 1,500 refugees have fled across the border to Thailand, and aid officials fear others will follow in coming months to swell the more than 140,000 already in Thai refugee camps.

Another congressman, Republican Joe Pitts, said the latest reports suggest the junta was actively hunting down more than 2,000 of the displaced.

“The thugs of Rangoon are on a violent rampage,” he said, referring to Yangon, the country’s former capital.

A leading British human rights advocate and House of Lords member, Baroness Caroline Cox, described the offensive as “the latest atrocities in a long litany of crimes against humanity” by the junta.

The military-run government has denied any human rights violations against ethnic minority groups, including the Karen, which it blames for a spate of recent bombings in the country.

“There is no offensive against the Karen National Union but security measures have been taken and cleaning-up operations are being conducted in some areas where (KNU) terrorists are believed to be hiding,” Information Minister Brig. Gen. Kyaw Hsan told reporters in Yangon earlier this month, referring to the main Karen rebel group.

But observers say the scale of the attacks is the largest since a major offensive against the Karen in 1997, and suggest that the military is trying to secure the hinterland east of the country’s newly established capital of Pyinmana.

“They don’t want the KNU near their new capital,” said KNU General-Secretary Mahnshar Laphan.

Myanmar’s military regimes, which first came to power in 1962, waged war against numerous ethnic insurgent groups seeking autonomy until a former junta member, Gen. Khin Nyunt, negotiated cease-fires with 17 of them.

But his ouster in 2004 reinforced hard-liners within the ruling junta and “resulted in increasing hostility directed at ethnic minority groups,” U.S.-based Human Rights Watch said in its 2006 report.

The KNU are the largest of the rebel groups still facing off against the regime’s 500,000-strong military.

The violence of recent years has spawned an estimated one million internal refugees and accelerated an exodus to neighboring countries.

A recent report by Free Burma Rangers said the Myanmar army destroys any Karen homes it comes across. “If civilians are encountered on the way, they shoot at them,” it said.

In one incident described in the report, Myanmar soldiers killed Saw Maw Keh as he was carrying his 80-year-old mother up a steep hill in western Karen State this month. The two were gunned down at point-blank range by soldiers near their village, which was being abandoned in face of an attack.

Nearby, a still unidentified villager was found with an eye gouged out and his nose cut off, one of the incidents of torture the group has documented with graphic photographs and video.