Afghan government spokesman Jawed Ludin says the plot is just one example of recent violence by Pakistanis or Arab foreigners within Afghanistan.
Ludin's remarks echo an accusation made on 17 June by Khalilzad, who suggested that Pakistani forces are not doing enough to capture remnants of the Taliban regime within Pakistan's territory.
"It is very likely that [Taliban leader] Mullah Omar and other senior Taliban are in Pakistan," Khalilzad said. "Mr. Usmani, who is one of the Taliban leaders, [recently] spoke to Pakistani Geo TV at a time when Pakistani officials claimed they did not know where they were."
In a pre-recorded farewell speech to Afghans -- which was distributed to Afghan broadcasters by the U.S. Embassy today -- Khalilzad reiterated that the fight against the remnants of the Taliban and Al-Qaeda is not finished.
"There are still some security threats to the Afghan people from the enemies of Afghanistan within Afghanistan," Khalilzad said.
Indeed, since the spring thaw in March, fighting in the south and east of Afghanistan has increased dramatically. U.S. and Afghan officials say about 260 suspected militants and 29 U.S. soldiers have been killed in clashes during the last three months. More than 30 Afghan police and soldiers also have been killed, along with more than 100 civilians.
Suicide bombers have attacked a mosque in Kandahar and businesses in Kabul frequented by Westerners.
There has been a series of kidnappings and attempted kidnappings of foreign aid and construction workers. Election workers preparing for the parliamentary ballot in mid-September also have been attacked and killed.
"Afghanistan is still far from being a success," said Sam Zarifi, a researcher for Human Rights Watch -- one of many nongovernmental organizations concerned about increasing violence ahead of the parliamentary ballot. "The country is still suffering from major insecurity outside of the urban areas. Certainly, we've known that to be the case. But we've now seen -- even in Kabul -- some major attacks. And with parliamentary elections coming up, the overall outlook for the rest of the summer has now been put into doubt."
Zarifi warned that the threat of violence is not just from Taliban and Al-Qaeda fighters. In many parts of Afghanistan, he said, local militia commanders or warlords may use violence to pressure voters into supporting candidates from their factions.
"Because of the nature of parliamentary elections -- because of how intensely local they are and because of the role of the future parliament -- we expect these races to be very competitive, actually, in a lot of places in Afghanistan," Zarifi said. "We saw that with the presidential election [in October 2004], there wasn't as much competition. And even then, we had some serious cases of intimidation and efforts to sway the vote. We expect this to be much more serious with the parliamentary elections as different groups [attempt to gain political] power."
Khalilzad said Afghanistan has taken enormous strides against terrorism, extremism, and warlordism during the past three years. But in his farewell speech, he said those gains need to be consolidated through peaceful and secure legislative elections.
"During the parliamentary elections this year, I hope you will vote for the people who will take into account the national interests, unity, and solidarity of all the Afghan people -- and who have this ability," Khalilzad said. "This is now a good opportunity for Afghanistan."
Khalilzad said all Afghans should remember how the battles between rival Afghan militia factions destroyed large areas of Kabul and other parts of Afghanistan after the withdrawal of Soviet forces in 1989.
"Now is the time for friendship and reconciliation," Khalilzad said. "Don't allow fighting between the brothers of Afghanistan once again. Don't allow Afghanistan's enemies to use you for their own aims."
Taliban fighters on 19 June executed eight police officials among 31 captives that they took as prisoners recently in the southern province of Kandahar.
Taliban spokesman Abdul Latif Hakimi said they were executed on the orders of the Taliban's "Islamic court" because they had cooperated with the central government in Kabul.
Hakimi told RFE/RL that the other 23 captives were released after local tribal elders promised that the prisoners would never again fight the Taliban. In that interview, Hakimi referred to the country by its old Taliban name -- the Islamic Emirate of Afghanistan.
"At 4 o'clock [yesterday afternoon], the chief of the Mianshin District [of Kandahar Province] Malik Noor Mohammad was released through the mediation of local tribal elders -- along with 22 other prisoners who were being held in local prisons of the Islamic Emirate of Afghanistan," Hakimi said.
Afghan officials say 18 Taliban fighters and five Afghan government soldiers were killed in clashes early yesterday. Eleven of those Taliban fighters were killed after attacking a government office in the southern province of Helmand. The other seven were killed after attacking a police checkpoint in southern Zabol Province along a stretch of the Kabul-Kandahar highway.
Two Afghan police officers also were killed on 19 June when Taliban fighters attacked their patrol in Herat Province.
U.S. military officials say an air strike called in against a concentration of Taliban fighters in Helmand Province on 18 June killed about 20.
(RFE/RL's Kabul bureau and Afghan Service in Prague contributed to this report. Translations from Dari and Pashto by Ahmad Takal in Prague.)