Sharing the concern of the Border Security Force that it was impossible to stop infiltration completely, Home Minister Shivraj Patil said the geographical conditions along the Indo-Bangla border were challenging and "...it was very difficult to stop persons going from one part to another".

Guarding the Indo-Bangla border was a challenging task due to the riverine patches and hilly terrain, he said. BSF has already made it clear that it was difficult to check infiltration along the 4,086 km long because of the terrain.

"It is not possible to totally seal the (Indo-Bangla) border because the terrain is such. Some kind of porosity will always be there," BSF Director General A K Mitra said. Reacting to reports that Pak-based terrorists generally use the Indo-Bangla border to enter the country and such was the case in the Mumbai blasts also, Mitra said, "I read such reports in newspapers everyday that they came from Bangladesh, but nobody has ever said that I came through this part or this area."

He said Bangladeshis could also enter the country using valid documents. "They come through Petrapol and Benapol and do not go back. There are at least 1.5 lakh people who have come with valid documents and have not gone back," he added.

Elaborating the hardships faced by border guards, Mitra said Indo-Bangla border was "peculiar" as it dissects rivers, hills and even houses and to complicate the situation further Bangladesh acts as a breeding ground for terror groups.

Apart from the Pakistan-based outfits and the ISI, there were two groups in the ruling party, which were associated with fundamentalist groups. "All these groups together try to export people" into India, Mitra said.

The Director General also expressed concern over recent attempts by Pakistan-based terrorists to enter the country through low-key Rajasthan and Punjab sectors.

"There have been no reports of infiltration along the Indo-Pak borders in Punjab and Rajasthan in the last ten years. Recently there were two attempts and this is a matter of concern for us and we are taking it seriously," Mitra said. He said it was not clear yet whether the infiltration bids by Laskher-e-Tayiba militants were one-off incidents or it was the beginning of a trend.

Mitra was referring to the killing of three terrorists by BSF personnel in Rajasthan and the gunning down of one Lashkar militant in Punjab in September.