The oppposition controls less than 10% of Afghanistan
He met the former king in Rome after Zahir Shah offered to return home if his services were needed in the current crisis and Afghanistan's anti-Taleban alliance announced it was sending a delegation for talks.
"I'm convinced that his majesty can play a very important role in the coming months," UN envoy Francesc Vendrell said.
Zahir Shah, who has lived in exile since being ousted in 1973, told the Italian newspaper La Repubblica that he supported punishing any terrorists based in Afghanistan but opposed military action that hurt the Afghan people.
The 86-year-old also called for a convocation of an Afghan national assembly to decide his country's future.
Foreign interference
"I am ready to return to Afghanistan if it serves to help my people," the former king said.
Zahir Shah appeared to accept that Afghanistan-based militants were responsible for the attacks on America.
"We knew that the external forces present in Afghanistan would accomplish something terrible such as the attacks in New York and in Washington," he said.
"These people present in Afghanistan, with the help of strong foreign interference, have turned my country into a place in which terrorism has gained a foothold."
Zahir Shah has often been spoken of as a figure who could surmount Afghanistan's ethnic divisions, says the BBC's Frances Kennedy in Rome.
He recently broadcast a radio message to the country in which he appealed to their sense of patriotism to save their country.
New government
Correspondents say there is growing support within Afghanistan for his return.
The exiled monarch said that Afghanistan's future should be decided by its own people in a "grand assembly to decide without external influence".
The anti-Taleban Northern Alliance has sent a team of officials to Rome to see the ex-king and they are due to meet within days.
The delegation aims to discuss "the future structure of a government for Afghanistan" with Zahir Shah and other Afghan exiles, spokesman Sayed Najibullah Hashimi told Reuters news agency.
Such a government would be "broad-based" and "acceptable to all the world's countries", he added.