U.S. and British warplanes now can fly at all hours with minimal concern about air defense threats, Defense Secretary Donald H. Rumsfeld said Tuesday as a third day of airstrikes against Afghanistan began.

Sunday's and Monday's bombing campaigns were very successful, he said.

"We believe we now are able to carry out operations more or less around the clock," Rumsfeld told a news conference.

Appearing with Rumsfeld, Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said the first two days of airstrikes damaged or destroyed more than 80 percent of targets throughout Afghanistan.

"Our forces continue operations against the Al Qaeda network and those who support them," Myers said.

Rumsfeld declined to say what the next step in the military campaign would be.

He said "it's pretty clear" the Al Qaeda terrorist network and the Taliban rulers who shelter them are feeling pressure from the strikes.

Pentagon officials who assessed the damage warned that air action alone would not be enough to neutralize Usama bin Laden or the Taliban regime. 

"The best defense against terror is a global offensive against terror whenever it might be found," President Bush declared Monday. "On all efforts, on all fronts, we're going to be ongoing and relentless as we tighten the net of justice." 

U.S. officials said the strikes likely were to continue at least one more day, and that the targeting of any follow-up attacks would depend in part on the assessments made of the damage inflicted during the first two nights. 

Beyond that, British Prime Minister Tony Blair hinted the offensive would expand in time. Airstrikes "will be supported by other actions," he said. He did not elaborate, but the British defense ministry said ground operations were an option. 

American officials sounded a similar note. 

"It's unlikely that the airstrikes will rock the Taliban back on their heels," Rumsfeld previously told reporters. "They do not have high-value targets or assets that are the kinds of things that would lend themselves to substantial damage from the air."

Myers said success depended on weakening the Taliban, helping opposition groups, feeding starving Afghans and demonstrating that those who harbor terrorists will be punished — not counting the number of weapons fired and the number of targets hit. 

"The only way that the Afghan people are going to be successful in heaving the terrorist network out of their country is to be successful against ... that portion of Taliban and the Taliban leadership that are so closely linked to the Al Qaeda," Myers said. 

Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee, said after a Pentagon briefing Monday that Americans should not expect large numbers of U.S. troops to enter Afghanistan. 

"In terms of massive ground assaults, they may never occur," said Levin, adding that an internal coup or rebellion against the Taliban would help prove the point that the United States has no quarrel with Afghans or Muslims. 

An admiral aboard the aircraft carrier USS Enterprise, cruising in the Indian Ocean, said the military was making "extraordinary efforts" to limit collateral damage, i.e. civilian casualties. 

"Our objective is to terrorize the terrorists," said the officer, whom reporters were not allowed to identify. 

At the Pentagon, Rumsfeld concurred. 

"The cruise missiles and bombers are not going to solve this problem. We know that," the defense secretary said. "What they can do is to contribute by adding pressure, making life more difficult, raising the cost for the terrorists and those that are supporting the terrorists, draining their finances and creating an environment that is inhospitable to the people that are threatening the world." 

Early Tuesday jets bombed the Taliban stronghold of Kandahar in southern Afghanistan, Taliban officials said. Taliban soldiers replied with heavy anti-aircraft fire. There was no immediate confirmation from the Pentagon that the attacks were from the U.S.-led coalition that began bombing Afghanistan on Sunday, although they likely were. 

Pentagon officials said five long-range bombers and 10 sea-launched warplanes took part in Monday's strikes against military and terrorist targets at selected locations inside Afghanistan, and all returned safely. Although smaller than Sunday night's bombardment, the attack included the launch of 15 cruise missiles, launched from destroyers USS John Paul Jones and USS McFaul and one submarine. 

At least three bomb explosions reverberated through the Afghan capital of Kabul. Taliban gunners responded to the attack with a crackle of fire into the skies over the city. 

Targets in Monday's raids included areas around the capital, the Taliban's home base of Kandahar, and Afghanistan's north, where an opposition northern alliance is battling the Taliban, the Islamic movement that controls most of Afghanistan. 

A spokeswoman for the United Nations in Pakistan, Stephanie Bunker, said four workers for the Afghan Technical Consultants, which had an office in a village two miles east of Kabul, were killed in Monday night's attacks. Their office was not far from a Taliban communications tower that may have been a target. 

Also in Islamabad, Abdul Salam Zaeef, the Taliban envoy to Pakistan, told reporters the United States "is aiming firstly to hunt the sitting Islamic government in Afghanistan and then every committed Muslim, in the name of terrorism." 

"We ask America to produce solid proof instead of allegations, but America is sending warplanes, bombs and cruise missiles in place of evidence," he said. "This is open terrorism." 

Rumsfeld said Monday that the first barrage on Sunday had damaged or destroyed some of the Taliban's air defenses, air fields and training camps. He and other military officials said it was too early to tell just how much damage the two nights of airstrikes had done — especially to leaders of both bin Laden's Al Qaeda terrorist network and the Taliban. 

Pilots and their commanders on the USS Enterprise said they scored several direct hits, including a surface-to-air missile storage facility and a training camp. 

Along with Monday's bombing, C-17 cargo planes air dropped about 37,000 packages of food rations for displaced civilians in Afghanistan, Rumsfeld said. The military dropped about the same amount of food on Sunday, and officials said they expected to continue delivering food for at least several more days. 

In a related development, 1,071 additional members of the Army Reserve and Army National Guard were called to active duty as part of a mobilization authorized by Bush shortly after the Sept. 11 attacks. Among those called up are personnel who specialize in criminal investigation, infantry or special operations. 

In all, 27,025 reservists from 44 states, the District of Columbia and Puerto Rico have been called up. 

The Associated Press contributed to this report