U.S. military commanders have shed their early caution in striking some targets in Baghdad and have embarked on more aggressive air attacks that run the risk of larger numbers of civilian casualties, defense officials said yesterday.

The strikes, many of them against communication nodes, telephone exchanges and government media offices, appear to reflect a judgment that winning the war against Iraq will require more aggressive air attacks, particularly the systematic destruction of networks used by the Iraqi authorities to direct their forces.

The heavier bombardment of the capital mirrors the intensified pounding in the field of Republican Guard divisions positioned south of Baghdad against U.S. Army and Marine forces. Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, told a Pentagon news conference yesterday that the attacks have reduced two Guard divisions to below 50 percent of their initial combat capability, preparing the way for U.S. ground forces to punch through to Baghdad.

Myers said the Iraqi divisions -- known as the Medina and the Baghdad -- remain largely in a defensive posture. They have not retreated, he said, but they have dispersed "into neighborhoods and things like that." They have also received some reinforcements from other Republican Guard formations positioned farther north, he said, referring to the Hammurabi, Nebuchadnezzar and Adnan divisions.

"We're just grinding it out," another U.S. general said, describing the bombing campaign. He said about 150 strike aircraft were in the air continuously over Iraq to enable round-the-clock pummeling of Guard units.

Early hopes of a quick victory in Iraq had led American commanders to refrain from striking some targets in the Iraqi capital that war planners had predicted could result in high numbers of civilian deaths. Just how high was not disclosed, but high enough to place those targets in a category referred to by military officials as "high collateral damage," or HCD.

In the war's early days, intense but brief barrages targeted presidential palaces, intelligence headquarters and Republican Guard facilities in Baghdad. While the damage was extensive, much of it appeared largely symbolic -- an attempt less to terminate government functions than to signal the impending end of Iraqi President Saddam Hussein's rule.

Since Friday, however, the strikes have intensified in both their scope and impact. Cruise missiles and precision-guided bombs have destroyed at least eight telephone exchanges, knocking out local phone service. Repeated attacks on television and radio transmitters have left Iraqi officials struggling to keep broadcasting with mobile equipment. Among the sites hit was a communications dish outside of the Ministry of Information and a communications bunker under the Rashid Hotel, both considered especially sensitive.

"We've now hit all our HCDs," a senior defense official said yesterday.

Even with the bolder strikes, defense officials noted that U.S. commanders are continuing to exercise great prudence in attacking targets to keep civilian deaths and damage to public works as low as possible.

"We're using delayed fuses and taking other measures to ensure only the effects we want are achieved," said a senior officer involved in the targeting.

But ratcheting up the air violence, officials acknowledged, raises the prospect of greater collateral damage. In the first two days of bombing, Iraqi officials said only four people died in Baghdad. By contrast, in one 24-hour period over the weekend, the government said 68 people were killed in airstrikes.

Thwarting the ability of Iraqi leaders to communicate with each other and with forces in the field has proven particularly challenging because of the built-in resilience of the country's command-and-control network. The communications system is said by U.S. officials to consist of extensive backup networks, some linked by deeply buried fiber-optic lines. Iraqi television, by employing mobile dishes, has withstood multiple efforts to take it off the air through attacks on its main transmission facilities.

"This regime is very effective at building redundancies," Army Brig. Gen. Vincent Brooks, deputy director of operations for the U.S. Central Command, said yesterday at a briefing in Qatar.

Some of the latest destruction is being done by a novel mix of warplanes. On Saturday, U.S. commanders employed a combination of B-1, B-2 and B-52 bombers in attacking targets in Baghdad -- the "first time in military history" that the three types of bombers had been used to strike "the same geographical area at the same time," according to a statement by the U.S. operations center in Saudi Arabia where the air war is being run.

While broadening the target set, U.S. commanders have not stopped going after sites associated with Hussein and his family. Among the targets hit in Baghdad yesterday, U.S. officials said, was an office complex that serves as headquarters for the Iraqi National Olympic Committee, where Iraqi exiles contend Hussein's son Uday has a private jail and a torture chamber.

Since the start of airstrikes two weeks ago, the United States has fired more than 700 cruise missiles and used more than 9,000 bombs, the large majority of them laser- or satellite-guided munitions. The heavy reliance on precision weapons has raised concerns among some that U.S. inventories could soon be depleted. But senior defense officials expressed confidence yesterday that American forces would have more than enough bombs and missiles for the Iraq war.

The largest percentage dip has come in the stocks of sea-launched cruise missiles, which are down about a third as a result of the Iraq war. But officials said the number of targets against which such missiles are likely to be fired has vastly diminished.

Other precision munitions are likely to remain in greater demand, but their stocks are larger, the officials said. For instance, of 18,000 satellite-guided Joint Direct Attack Munitions at the start of the war, about 2,400 have been dropped by Air Force and Navy planes so far. Similarly, out of an initial inventory of 26,000 laser-guided GBUs, 1,300 have been expended in the war.

"We don't see a problem there," Air Force Secretary James G. Roche told a group of defense writers yesterday.

A member of Iraq's ruling Baath Party stands next to a bomb crater at the site of a Baghdad telecommunications building, which was hit in an airstrike Sunday.Smoke billows from an explosion that rocked Baghdad. U.S. commanders aim to systematically destroy Iraq's communications facilities.