Pakistan's interior ministry has called for the electoral commission to bar from politics a new party backed by an Islamist with a $10 million U.S. bounty on his head, a government document seen by Reuters showed on Thursday.

In a letter dated Sept. 22, the ministry recommended that the Election Commission of Pakistan reject the newly formed Milli Muslim League's (MML) application to become an official party as it is "affiliated" with Lashkar-e-Tayyeba (LeT), a militant group blamed for the 2008 Mumbai attacks that killed 166 people.

"The registration of MML is not supported," the ministry said in the two-page document.

Spokesmen for the election commission and the interior ministry acknowledged the correspondence and confirmed that the letter was authentic.

The United States has designated LeT founder Hafiz Saeed, who currently heads the Jamaat-ud-Dawa Islamic charity, a terrorist. It views him as the mastermind of the 2008 Mumbai attacks and has offered a $10 million reward for information leading to his imprisonment.

Saeed is currently under house arrest. Pakistan's reluctance to press charges against him has been a sore point in relations with Washington and India over the past decade.

The ministry said MML is "ideologically of the same hue" as LeT and its affiliated charities Jamaat-ud-Dawa (JuD) and the Falah-e-Insanyat Foundation (FIF).

Tabish Qayyum, a spokesman for the MML, said in a statement that the ministry's letter was unlawful.

"MML isn't a bus or truck which needs registration," he said, denying that MML had links with any banned militant group.

The ministry's stance appears at odds with what political sources and a retired army general have said is a plan proposed by the military's Inter Services Intelligence (ISI) to 'mainstream' some Pakistan-based anti-Indian militant groups as part of deradicalisation efforts by bringing them into politics.

The interior ministry's letter was written a week after MML caused a stir by winning 5 percent of votes in a parliamentary by-election in Lahore on Sept. 17.

The document said foreign countries have raised diplomatic objections to MML's existence and the interior ministry has sought the opinions of intelligence agencies on the group.

One of the agencies, the ministry said, has warned against letting proscribed and monitored organizations enter politics with a view to gaining legitimacy.

The interior ministry said the security agency has informed it that "given the clamour, philosophy, outreach and modus operandi to operate, it is difficult to believe that MML will tread its own path completely at variance with its mother organization."

"Therefore, they have recommended that since the registration of such groups would breed violence and extremism in politics, as such registration of such groups be avoided."

In the Lahore by-election, Yaqoob Sheikh, who swears loyalty to Saeed, stood as an independent candidate but was backed by MML and had Saeed's colleagues running his campaign.

Saeed's portraits adorned posters promoting Sheikh, who the United States has also designated a terrorist and a senior LeT commander.