US Secretary of State Rex Tillerson has undermined President Donald Trump’s promise to win the Afghan war and defeating the Taliban, saying Washington might not succeed in the war.

“I think the president was clear this entire effort was intended to put pressure on the Taliban, to have the Taliban understand that you will not win a battlefield victory,” Tillerson said during a press conference Tuesday afternoon. “We may not win one, but neither will you.”

His remarks came one day after Trump said he would prolong the military intervention in the South Asian country, which he once described as a “complete waste.”

Tillerson also stepped up pressure on the neighboring Pakistan over its perceived support for the Taliban in Afghanistan.

He pointed to the fact that the US is a key ally of Pakistan and that Islamabad enjoys a special status as a non-NATO partner which receives billions of dollars in aid.

US military personnel listen to President Donald Trump deliver remarks on America's involvement in Afghanistan at the Fort Myer military base, on August 21, 2017, in Arlington, Virginia. (Photo by AFP)

These, however, "could be on the table for discussion if in fact they are unwilling to change their posture or change their approach to how they are dealing with the numerous terrorist organizations that find safe haven in Pakistan,” Tillerson added.

"It is in Pakistan's interest to take those actions."

Trump also said Monday that Washington can no longer remain silent towards Pakistan’s “safe havens for terrorists,” threatening Islamabad to cut the “billions and billions of dollars” of US aid.

The United States -- under Republican George W. Bush’s presidency -- and its allies invaded Afghanistan on October 7, 2001 as part of Washington’s so-called war on terror.

Trump who had previously called for withdrawal of American troops from Afghanistan argued that his "original instinct was to pull out," but that he was convinced by his national security team to take on the Taliban militants.