Guwahati city superintendent of police Hiren Nath claimed that 50 militants of the Harkat group were "hired" by the ULFA last week to carry out attacks in the state.
The growing links between ULFA and Maoist militants had also caused concern among security agencies.
There were reports that the ULFA had set up camps in Nepal with the help of Maoists, but the security forces were yet to get any conclusive evidence in this regard, he said.
Police said top ULFA leaders had meetings with Maoist leaders with whom they first came into contact in Bhutan where there is a sizeable population of Nepali community.
Senior ULFA leaders visit Nepal quite often and it was believed that the outfit was currently using that country as a shelter point after Bhutan's operation against them, police said.
Security agencies apprehended that the ULFA might be procuring arms from Nepal but "we are yet to get any substantive evidence to prove that", the sources said.
The frequency of movement of Maoist militants through North Bengal is also a cause of serious concern as the leaders of North-East based militants frequenty visit that area, the sources said.
This year alone ten Maoist leaders and hardcore activists were arrested in North Bengal where they came mainly for medical treatment and purchase publicity materials.
The Maoist ultras have also been using the North Bengal corridor to go to other states as Bihar and Uttar Pradesh where they have "sympathisers", they added.
The ULFA also operated together with the Kamtapur Liberation Organisation (KLO) in the North Bengal area and taken shelter there on their transit route to Bangladesh, Bhutan and Nepal, police said.
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