UPDATE 1-Militants pouring in from Afghanistan: Pakistan

(Updates fighting, adds U.N. comment on refugees)

By Zeeshan Haider

RAWALPINDI, Pakistan, Sept 29 (Reuters) - Militants battling Pakistani forces are getting weapons and reinforcements from Afghanistan, security officials said on Monday, vowing no let-up in their offensive in the northwest.

Government forces launched an offensive in the Bajaur region on the Afghan border in August after years of complaints from U.S. and Afghan officials that Taliban insurgents in Afghanistan were getting help from Pakistani border areas such as Bajaur.

Now the tables have turned and the militants locked in heavy fighting with Pakistani forces are getting help from the Afghan side of the border, officials said.

"The Pakistan-Afghan border is porous and is now causing trouble for us in Bajaur," a senior security source in the military told a news briefing.

"Now movement is taking place to Pakistan from Afghanistan," said the official, who along with a colleague at the briefing, declined to be identified.

The officials did not blame the Afghan government for sending militants across the border but called on Kabul and U.S.-led forces in Afghanistan forces to stop the flow.

Bajaur is the smallest of Pakistan's seven so-called tribal agencies, semi-autonomous ethnic Pashtun tribal regions.