Afghan and Western officials say Tehran has quietly increased its supply of weapons, ammunition and funding to the Taliban, and is now recruiting and training their fighters, posing a new threat to Afghanistan's fragile security, the Wall Street Journal reported on Friday.

The paper quoted these official as saying that Iran's strategy in backing the Taliban is twofold countering US influence in the region and providing a counterweight to Daesh's move into the Taliban's territory in Afghanistan.

The Taliban's aggressive military push and the new momentum toward peace negotiations between them and Kabul also raises the possibility that some of their members could eventually return to power.

"Iran is betting on the re-emergence of the Taliban," said a Western diplomat. "They are uncertain about where Afghanistan is heading right now, so they are hedging their bets."

Iranian officials didn't respond to requests for comment, but Tehran has repeatedly denied providing financial or military aid to the Taliban in conversations with Afghan and Western officials. "Whenever we discussed it, they would deny it," a former senior Afghan official said.

The developing Iran-Taliban alliance represents a new complication in Obama's plans for both the Middle East and the future of Afghanistan, where the US has been working to curb the Taliban's role ahead of a planned withdrawal of all but 1,000 US troops at the end of his presidency in 2016. At its peak in 2011 there were 100,000 US troops.

The White House-supported international nuclear talks with Iran that are scheduled to finish this month face world-wide criticism for potentially setting up a new regional dynamic in which Tehran, unfettered by punitive economic sanctions and flush with new resources, would be able to pursue an activist agenda through its proxies in and around the Middle East. Tehran’s growing ties to the Taliban is another sign of that, these critics say.

Sen. John McCain, chairman of the Armed Services Committee, said Iran's increased support to the Taliban is a continuation of its aggressive behavior in Yemen, Syria, Iraq and Lebanon. "This is further evidence of the administration’s continued willful disregard for the facts on the ground in light of Iranian aggression in the region," he said.

The paper added that the Iranian Revolutionary Guard Corps have been delivering weapons to the Taliban since at least 2007, according to an October 2014 report by the US Department of Defense.

Iran's alliance with the Taliban took a new turn in June 2013 when Tehran formally invited a Taliban delegation to participate in a conference on Islam and to meet senior Iranian officials.

By the fall of that year, Afghan security officials said they had clear evidence that Iran was training Taliban fighters within its borders. Tehran now operates at least four Taliban training camps, according to Afghan officials. They are in the Iranian cities of Tehran, Mashhad and Zahedan and in the province of Kerman.

"At the beginning Iran was supporting Taliban financially," said a senior Afghan official. "But now they are training and equipping them, too."