WASHINGTON -- U.S. and Iraqi security forces now control about half of Baghdad, and as pressure on the insurgents increases some are fighting back with remarkable tenacity, a senior U.S. commander said Friday.

"This is a skilled and determined enemy. He's ruthless. He's got a thirst for blood like I've never seen anywhere in my life," said Army Maj. Gen. Joseph F. Fil Jr., the 1st Cavalry Division commander who has been in charge of military operations throughout the capital since late last year.

He spoke to reporters at the Pentagon via satellite video link from his headquarters in Baghdad.

At a separate news conference later, Defense Secretary Robert Gates said he is pressing for faster production of a new military vehicle designed to provide better protection against the roadside bombs, which the military calls improvised explosive devices, or IEDs, that are the leading killer of U.S. troops in Iraq.

Gates said he is demanding an accelerated effort to build a mine-resistant armored vehicle, called the MRAP, and get it to Iraq in large numbers to replace the more vulnerable Humvee utility vehicle used by soldiers and Marines.

"The way I have put it to everyone is that we have to look outside the normal bureaucratic way of doing things, and so does industry, because lives are at stake," Gates said. "For every month we delay, scores of young Americans are going to die."

Fil said security forces now control 48 percent to 49 percent of the 474 neighborhoods in Baghdad.

That is up from 19 percent in April, he said. Two weeks ago his boss, Lt. Gen. Raymond Odierno, said about 40 percent of the city was under control.

Fil defined "control" as "where we have our security forces there and we're denying that space to enemy forces."

He said that U.S. and Iraqi forces are conducting clearing operations in 36 percent of the capital's neighborhoods -- about the same percentage as in April. In neighborhoods that are neither under control nor in the process of being cleared, coalition forces are "disrupting" insurgent forces, Fil said.

He declined to predict how long it would take to get the entire capital under control.

Asked about an attack Thursday that killed five soldiers under his command and wounded several others, Fil said they were hit with a deeply buried roadside bomb, followed by small arms and rocket-propelled grenade fire.

"We thought it did show a level of sophistication that we have not often seen so far in this campaign," he said, referring to the insurgents' ability to mount a multi-pronged attack. He said it happened in East Rasheed, a southern sector of Baghdad, where fighting has proven especially tough.

At his news conference, Gates referred to Thursday's attack, saying it was troubling in many respects.

"It was a more sophisticated attack in terms of the way they planned it," he said without elaborating. "And we're seeing some more of that" as U.S. forces press their offensive in and around Baghdad.

Fil said U.S. and Iraqi forces in East Rasheed have encountered a "very strong" cell of the al-Qaida in Iraq group. "They're running out of maneuver space, and they are starting to fight very hard, and that's what we saw yesterday," he said.

On Capitol Hill, leading Democrats said they are planning to continue efforts to force a change in President Bush's Iraq policy.

"People are down on government for a lot of reasons, but the big reason is the war in Iraq," said Senate Majority Leader Harry Reid, D-Nev.

In July the House and Senate will each vote on a proposal written by Sen. Carl Levin, D-Mich., that would order troop withdrawals within four months and set the goal of completing the pullout by April 2008. Under the bill, troops could remain in Iraq to target terrorists, train Iraqi security forces or protect U.S. diplomats.

Other proposals that will be voted on by one or both chambers would rescind congressional authorization for the Iraq invasion, ban permanent bases in Iraq, cut funding for combat and restrict the length of combat tours.

"We have many arrows in our quiver, and we are sharpening them," said House Speaker Nancy Pelosi, D-Calif.