KABUL, Afghanistan - Amid a spike in Afghan and American deaths in southern Afghanistan, U.S. Army Gen. Stanley McChrystal, the top NATO commander in the country, conceded Thursday that the military push to secure the Taliban's spiritual capital would take longer than anticipated.

A series of obstacles has slowed the opening stages of the highly anticipated summer offensive to defeat Islamic extremists and bolster the Afghan government in Kandahar province.

A Taliban intimidation campaign has undermined the American-led effort to build a functional Kandahar government.

Afghan security forces are taking longer than planned to prepare for the summer campaign.

The Afghan government's efforts to install respected local leaders have lagged.

And U.S. Marines in neighboring Helmand province have yet to gain the upper hand against resurgent Taliban fighters.

On Thursday, McChrystal, the head of U.S.-led military forces in Afghanistan, said the high-profile Kandahar operations, which once were expected to produce significant results by mid-August, would now continue well into the fall - and beyond.

"I do think that it will happen more slowly than we had originally intended," McChrystal told reporters while visiting Brussels, Belgium, for a meeting of NATO leaders. "But I don't think that's necessarily a bad thing. I think it's more important we get it right than we get it fast."

McChrystal said that "more prep" would have helped the U.S.-led military to transform the Taliban sanctuary of Marjah more quickly into a model for his counterinsurgency strategy.

"As we did it, we found that it's even more complex than we thought, and so we need to educate ourselves from that and do it even better in Kandahar," McChrystal said.

His comments came three weeks after he warned American and British strategists in Helmand province that Marjah was becoming a "bleeding ulcer" that needed immediate attention.

After McClatchy reported that, an allied military spokesman in Afghanistan asserted that McChrystal had been misinterpreted and said: "The essence of the comment is not that Marjah itself is going badly. . . . It's largely on track. It's that it's misperceived as going badly."

The challenges facing McChrystal in Kandahar were evident Thursday as Afghan families began burying more than 40 men and boys who were killed by a suicide bomber who targeted a wedding party Wednesday night in the province's Arghandab district.

Afghan President Hamid Karzai denounced the attack as a "crime of massive, inhuman proportions."

"Weddings all over the world are not only good occasions but occasions of sanctity," Karzai said during a news conference with British Prime Minister David Cameron, who was making his first visit to Afghanistan since taking office. "For a suicide bomber to go and kill is not only against Islam, it is an act against the whole of humanity."

Survivors of the attack accused the Taliban of dispatching the suicide bomber because the groom was an Afghan police officer and member of a local militia that battled Taliban fighters earlier in the week.

The Taliban condemned the attack and denied responsibility.

June has also proved to be a deadly month for the U.S.-led military in Afghanistan. At least 30 coalition troops have been killed this month, most of them in the last week.