The decree - which is said to come from Mullah Mohammed Omar - calls for a jihad against both U.S. troops and Afghans who work with them.
Omar's decree carries the signatures of 600 Islamic clerics reminding the faithful of their duty to wage jihad.
"Whenever the non-Muslims attack a Muslim land it is the duty of everyone to rise against the aggressor," reads posters that are reportedly authored by Omar and openly displayed in towns and villages in eastern Afghanistan and in the tribal regions of Pakistan.
"We were blamed for Osama bin Laden because they said he was a terrorist and he was taking shelter with us. But what is the fault of Iraq? Iraq has no Osama bin Laden in its country," the black and white poster says.
International peacekeepers in the Afghan capital of Kabul and the U.S. military headquartered north of Kabul at Bagram Air Base say they don't expect the war in Iraq to cause a deterioration in their security situation.
Sunday's rocket attack, which did not cause any deaths or injuries, was one of a series of incidents observers say could indicate a resurgence of activity by holdout Taliban fighters.
On Saturday, four gunmen on motorcycles ambushed a U.S. military reconnaissance patrol in southern Afghanistan, killing two U.S. soldiers and wounding another American and three Afghan soldiers.
The ambush was the first fatal encounter for U.S. forces in Afghanistan since December. It came just two days after a Red Cross worker was killed, also in southern Afghanistan.
U.S. special operations soldiers backed by air support, and joined by about 1,000 Afghan troops, battled about 100 Taliban fighters over the weekend in southern Uruzgan province. At least 15 Taliban fighters were killed.
Taliban loyalists hiding in Pakistan's tribal regions told The Associated Press in recent interviews that they have regrouped and built alliances with al-Qaida and rebel warlord Gulbuddin Hekmatyar's Hezb-e-islami. They said the next phase would be hit-and-run attacks.
Small pamphlets that had been circulated by Taliban, Hekmatyar and al-Qaida operatives have given way to poster-size warnings to international forces.
The latest warning, a 24-inch by 18-inch poster, combines verses from the Muslim holy book, the Quran, explaining jihad or holy war. It is written in Pashtu, the native language of Afghanistan's majority Pashtuns and the ethnic group from which the Taliban foot soldiers were drawn.
"Being a leader of Muslims, I accept that jihad against U.S. troops is our duty and everyone who assists Americans in executing their mission is liable to death," Omar warns.
"There are only two symbols left in the world today: One is Islam, which is a religion of peace, and the other symbol is (U.S. President George W.) Bush, who is a symbol of terror and hatred," Omar said.