Kenyan authorities are questioning a legislative employee about a suspected plot to attack parliament in Nairobi, police and lawmakers said on Monday, amid concern after assaults by Islamist militants.

Ali Abdulmajid, a long-serving parliamentary employee, was arrested on Sunday and held in the Kenyan capital after intelligence reports linked him to the plot, Ndegwa Muhoro, Director of the Criminal Investigations Department, said.

"He is in our custody assisting us with our investigations," he added.

Hundreds of Kenyans have died in five major attacks in the last two years, including a days-long siege at Nairobi's Westgate mall in late 2013 that left 67 people dead and a pre-dawn attack at Garissa university earlier this month that killed 148 people, most of them college students.

Somali Islamists al Shabaab have claimed responsibility for the attacks, which it says were retaliation for Kenya contributing troops to an African Union-led force in Somalia that has made significant gains against the militants.

A confidential report, which was issued last week by the Kenyan government and seen by Reuters, said al Shabaab had dispatched spies to "launch more high-scale attacks in the country on unspecified dates".

Targets included Nairobi Pentecostal Church, the University of Nairobi and parliament, said the report that appeared to directly refer to Abdulmajid.

"The group intends to use an operative, who is affiliated to Pumwani Riyadh Mosque (and) who is also a staff member at the Senate, to actualise the attack," the report said.

Police said Abdulmajid was known to be active with the Pumwani Riyadh mosque, which Kenyan authorities have in the past said could have links to terror funding. In addition, he was named in a U.N. Monitoring Group on Eritrea and Somalia 2011 as having possible links to terror activities, but was not charged.

Senator Hassan Omar, who represents the coastal area of Mombasa and is an acquaintance of Abdulmajid, said he doubted there was any basis to the accusations.

"Lately, they're just becoming reckless," Omar said of the government. "All it requires to be arrested now is to put on the Muslim garb."