WITH U.S. FORCES, SAUDI ARABIA -- Newly arrived U.S. Marines and British forces have moved northward in anticipation of an offensive against Iraqi forces on the Saudi Arabian border, but three U.S. Army tank divisions are not yet fully deployed, casting some doubt on U.S. readiness for war.

On Tuesday, British Prime Minister John Major visited some of the 30,000 British soldiers of the 1st Armored Division deployed in the northern reaches of the Arabian Desert. Earlier this week, the newly arrived 4th Brigade announced full operational readiness, bringing overall British strength to three tank battalions and three armored infantry battalions.

The American 2nd Marine Division also has completed its deployment and is conducting final training exercises to calibrate its weapons and move into forward positions in anticipation of the Jan. 15 U.N. deadline authorizing the use of force to dislodge Iraqi troops from occupied Kuwait. With the new arrivals, the 1st Marine Expeditionary Force has approximately 60,000 troops in Saudi Arabia.

Equipment and soldiers from the U.S. Army's VII Corps, however, continued to stream into Saudi ports and airports, moving into the desert to prepared campsites where training and preparations are underway. Once in place, the Army will have three full armored divisions and two mechanized infantry divisions in Saudi Arabia.

Army sources here have been reluctant to provide details of the VII Corps deployment but acknowledged that none of its three divisions will be fully deployed, trained and ready by Jan. 15.

Anticipating these delays, Army Lt. Gen. Calvin A. H. Waller, deputy commander of coalition forces in Operation Desert Shield, told reporters Dec. 19 that U.S. troops "will not be ready for combat activities" by Jan. 15 and gave the date of final preparedness as "sometime between the 15th of January and the middle of February." Other defense sources have insisted, however, that this would not hamper U.S. readiness to launch an offensive against Iraq any time after the mid-January deadline.

Still, the delays mean that the Army may not immediately have full use of two of its armored divisions and one mechanized infantry division if there is an early confrontation with Iraq, a battle in which tanks and mechanized infantry are expected to play a primary role.

Iraq is believed to have about 6,500 tanks and armored vehicles massed in prepared fortifications on the Saudi border with occupied Kuwait and in Iraq. U.S. ground forces currently have about 3,000 tanks and armored personnel carriers to challenge them, according to American officials.

With the arrival of the new year, U.S. and allied units have begun to close down many of their rear-echelon encampments and headquarters staging areas, moving northward deep into the desert wilderness close to the Saudi frontier.

Convoys of trucks, many driven by civilians under military contract, have been ferrying everything from main battle tanks to bottled water and lumber into the desert to forward positions well within striking distance of the Kuwaiti border.

At one wind-blown stretch of sand dunes earlier this week, British Brig. Christopher Hammerbeck, commander of the 4th Brigade, displayed an array of desert-painted tanks, armored vehicles, mobile artillery and special attack vehicles, pronouncing his command "complete in theater and operationally ready for its task."

He said the 4th Brigade had been deployed for 10 days and had fully adapted to the desert environment. Equipment had been tested and logistics problems resolved, he said.

In the early stages of Operation Desert Shield, British forces came under operational control of the U.S. Marines, but this arrangement appears to have changed with the arrival of the 4th Brigade and the headquarters component of the 1st Armored Division.

It is now expected that the division will fight as an independent command reporting directly to U.S. Army Gen. H. Norman Schwarzkopf, commander of all Desert Shield coalition forces. Hammerbeck did not respond directly to this question except to say the command question "has been resolved" and to confirm that "we will be a part of, and under the operational control of, American forces."

The 4th Brigade displayed an elaborate array of bridging, trench-breeching and sapping equipment, an indication that the British may be preparing for possible frontal attacks against Iraqi border fortifications, a role that the Marines also are expected to play.

Cpl. Christopher David, a member of the British 39 Field Squadron Engineers and an expert in demolitions and barrier clearing, demonstrated how one vehicle could spill a five-foot-high bundle of plastic pipes known as a "fascine" into a tank trap, creating a bridge for friendly forces.

He also said the vehicle was equipped with a plow for mine-clearing and with "vipers," rocket-propelled explosive cables that can blow a two-lane highway through a minefield.

On Tuesday, the 1st Armored Division wound up what may be its final series of demonstrations and photo opportunities with a one-day visit from Prime Minister Major, on a three-day tour of the Middle East.

The 47-year-old prime minister, carrying a gas mask looped over his shoulder, rode in a Challenger tank and encouraged the troops with British understatement: "Unless Saddam Hussein gets out of Kuwait and goes back to Iraq, we will invite you to forcibly remove him."

With the British on their right flank, U.S. Marines in the last week have abandoned many of their base camps to move forward into assault positions close to the Kuwaiti border, bringing with them a logistics train designed to supply them for a possible assault on Iraqi fortifications and beyond.

Truck convoys drove through the northern desert on a cross-hatched pattern of makeshift, rutted-sand thoroughfares, their tires sending plumes of dust soaring into empty blue skies.

Throughout their northern operations area, the Marines gave the impression of buttoning up their defenses, plugging the last holes in their supply lines and fine-tuning vehicles and equipment. Camps were darkened at dusk, and sentries patrolled secure perimeters on foot and in all-terrain vehicles equipped with machine-gun turrets.

"The tourist season is about to close down," Col. Tom Hampton, 43, of Carterville, Ill., commander of a logistics base, told visiting reporters. "From now on, you come up here to stay."