CHICAGO — Pakistan is the "main threat" to Afghanistan and its intelligence service is "the biggest danger" to the government in Kabul, a US senator recently returned from the region warned Tuesday.

Republican Senator Mark Kirk, just returned from a two-week stint serving as a US Naval Reserve Intelligence Commander, charged in a speech that the militant Haqqani network is "backed and protected" by Pakistan's powerful spy agency.

"Statements by Pakistani government officials to the contrary are direct lies," Kirk said in a transcript of his address provided by his office.

Pakistan's Inter-Services Intelligence (ISI) has long been held to keep contact with militants operating in the wild tribal areas along the Pakistan-Afghanistan border who plague US-led forces fighting the Afghan war.

"Let me be clear: many Americans died in Afghanistan because of Pakistan's ISI," said the lawmaker, who declared it was time to rethink potentially "counter-productive" US aid to Islamabad.

"Pakistan has become the main threat to Afghanistan. Pakistan's intelligence service is the biggest danger to the Afghan government. It is also a tremendous threat to the lives of American troops," said Kirk.

Affiliated with the Taliban, the Haqqani network is considered the most dangerous enemy of US troops in eastern Afghanistan.

It was founded by Jalaluddin Haqqani and is run by his son, Sirajuddin, both of them already designated "global terrorists" by Washington.

The network has been blamed for some of the deadliest anti-US attacks in Afghanistan, including a suicide attack at a US base in the eastern province of Khost in 2009 that killed seven CIA operatives.

"It has become the most dangerous, lethal and cancerous force in Afghanistan," said Kirk, who declared he had had a change of heart about Pakistan as a result of his service.

"I first served as a reservist in Afghanistan in 2008. I believed that Pakistan was 'complicated,' that 'we have many interests there' and that we must advance 'diplomatically.' I no longer agree with that," he said.

"In such an environment, and with our deficits and debt, aid to Pakistan seems naive at best and counter-productive at worst. I am seriously reconsidering and rethinking how well aid to Pakistan served us," he said.