KABUL — Three U.S. senators visiting Kabul said Sunday they are worried that President Barack Obama's planned withdrawal of 33,000 American troops by September 2012 could undermine Afghan morale, embolden the insurgency and hamper efforts to defeat Taliban fighters.

John McCain, Joe Lieberman and Lindsay Graham said they are heartened by the progress of Afghan security forces, but worry that Obama's withdrawal plan could deplete American military strength before dealing a decisive blow to the Taliban, especially in eastern Afghanistan. That part of the country is a haven for the Afghan and Pakistani wings of the Taliban, and al-Qaeda affiliates.

"I believe that the planned drawdown is an unnecessary risk and that's why there was no military leader that recommended it," McCain, a Republican from Arizona, said during a stop at the Kabul headquarters of the foreign military contingent, called the International Security Assistance Force.

Lt. Gen. John R. Allen, a Marine general expected to carry out the president's drawdown order, has said the schedule is a bit more aggressive than the military had anticipated. Allen has cautioned that successfully winding down the war will require new progress on a wide front, including more help from allies and less Afghan corruption.

McCain said he's concerned there may not be enough American troops for a move from southern Afghanistan to the east to "finish the job there." NATO has deployed the bulk of its forces to Helmand and Kandahar, two southern provinces where Afghan Taliban influence is strong, but international terrorist groups are less influential.

McCain said the drawdown will deprive NATO "to a significant degree" as it attempts to pacify eastern Afghanistan next summer.

Graham, a Republican from South Carolina, cited gains in Afghan security force recruitment and capability and said he was optimistic that native forces would soon be ready to take over security. But Graham also worried Obama's withdrawal plan may reduce U.S. troop strength in Afghanistan too quickly.

"Withdrawal is what the enemy hopes to hear," said Graham. "Our goal is to make sure that the enemy doesn't hear withdrawal and the Afghan people don't hear withdrawal."

Lieberman, an independent from Connecticut, praised increased security in Afghanistan and said the situation was improving, but he noted that spectacular violence in Afghanistan will likely continue for some time.

"Unfortunately this is the age in which we live," Lieberman said. "Extremist terroristst who are crazy enough to want to blow themselves up and other people will continue to have the capacity to do that, not only in Afghanistan and Iraq, but also in the United States of America as we have seen."

Shortly before the senators' news conference in Kabul, an improvised bomb exploded on the other end of the capital, wounding three Afghan policemen, the Afghan Interior Ministry said. Insurgents have focused many of their attacks on Afghan security forces to undermine their development and NATO's plans to transfer security operations to Afghan control.