TEHRAN, Iran -- Iran's supreme leader told the country's military and its top civilian officials Tuesday to be ready to carry out "speedy, timely and forceful implementation" of coming decisions in a crisis over Afghanistan.

The directive, by Ayatollah Ali Khamenei, stopped short of a full military alert, senior Iranian officials said. But it was enough to prompt a furious response from the militant Taliban movement that governs Afghanistan, which vowed to strike back if Iran makes good on repeated threats to use military force.

A Taliban spokesman, Wakil Ahmed, told the Afghan Islamic Press, a news agency: "Iran must know that if the soil of Afghanistan is attacked, we will target Iranian cities and the entire responsibility will rest with Iranian authorities."

Tehran is still seething over the recently confirmed killings of at least eight Iranian diplomats and a journalist by Taliban forces. Seven of the bodies were returned to Tehran early Tuesday morning and were received in an emotional ceremony at the airport.

Tuesday's call for heightened readiness was clearly intended to increase the pressure on the Taliban. Iranian officials have also said the Taliban -- who practice a purist form of Sunni Islam -- must be held accountable for a campaign of atrocities against Shiite Muslims, who make up about 15 percent of Afghanistan's population but 95 percent of Iran's.

United Nations officials have now validated at least some of those accusations, initially made by international human rights groups, and say that Afghan refugees reaching Pakistan have given credible and chilling accounts of the deliberate killings of large numbers of Shiite civilians in the northern Afghan town of Mazar-i-Sharif in August.

A spokesman for the U.N. High Commissioner for Refugees said in a telephone interview Tuesday that refugees who fled the town after it was overrun by the Taliban on Aug. 8 have "very consistently" provided reports of house-to-house searches that targeted Shiites for execution.

The spokesman, Rupert Colville, said the refugees have also reported that bodies were left in the street for several days, and that some Shiites died of heat or suffocation after being loaded into containers that were hauled away by truck.

Iran has condemned the killings as genocide and warned that more killings may be under way in the central Afghan city of Bamiyan, which the Taliban captured over the weekend.

Taliban spokesmen have repeatedly denied that their forces committed ethnic reprisals, and in recent days the Taliban radio has repeatedly broadcast instructions from commanders that civilians in Bamiyan should not be harmed.

Iran has long been at odds with the Taliban, and it has been a major supplier of logistical and military support to the opposition forces who have been trying to prevent the Taliban from consolidating their control.

But with the capture in the last five weeks of Mazar-i-Sharif, the opposition's northern headquarters, and Bamiyan, its last remaining stronghold, the Taliban have now also taken control of the airstrips and land routes that Iran has used to provide its indirect support.

It is the deaths of the diplomats, though, that has stirred Iranian passions, creating what Tehran Mayor Gholamhossein Karabaschi described in an interview Tuesday as "a very dangerous time."

Another senior Iranian official, who would speak only on condition of anonymity, said in a separate conversation, "The honor of Iran has been damaged, and the people expect revenge."