U.S. officials continued to hold out hope yesterday that Turkey will agree to let U.S. ground forces use its bases for an invasion of Iraq, even as military planners prepared to shift to an alternate plan for occupying northern Iraq.

With about two dozen military cargo ships in the eastern Mediterranean standing by to unload tanks and other weaponry in Turkey for the U.S. Army's 4th Infantry Division, Pentagon officials have been saying for days that virtually no time remained before the ships would have to be redirected to Kuwait and the war plan changed.

But no such order came yesterday, according to several military officials, suggesting some slight flexibility still existed in the Pentagon's timetable. The delay also appeared to reflect the reluctance of the Bush administration to give up on the Turkish option, for both military and political reasons.

Militarily, while alternatives exist for placing U.S. forces in northern Iraq -- essentially by airlifting lighter Army and Marine units to key locations -- these options pose greater logistical challenges for American troops and carry heightened risks of U.S. casualties, officials and analysts said.

Politically, too, a U.S.-led invasion of Iraq without Turkey would not only substantially weaken administration claims of international support but also do critical long-term damage to relations with an important NATO and Muslim-populated ally, analysts said.

Several U.S. officials portrayed the administration as still evaluating Turkey's political situation after Friday's parliamentary vote that failed to approve access to Turkish bases. Although expressing deep disappointment and frustration with the lack of Turkish approval, they noted the possibility that another vote could open the way to U.S. troops, although no new vote has been scheduled.

Gen. Tommy R. Franks, the head of U.S. Central Command who would lead any U.S. war in Iraq, was reported by aides yesterday to have not yet ruled out the use of Turkish bases. Under his preferred war plan, U.S. ground forces would enter Iraq both from Turkey in the north and Kuwait in the south, opening two fronts and stretching Iraqi defenses.

But one defense official said more intense work had begun on other options -- the kind of detailed preparation that usually precedes new orders, the official added. Several senators, speaking on television interview shows yesterday, expressed shock at Turkey's vote and sounded pessimistic about the prospects for a reversal.

"It's a huge setback for our purposes. It stunned me," Sen. John D. Rockefeller IV (W.Va.), the ranking Democrat on the intelligence committee, said on CNN's "Late Edition." "We spent the last 50 years defending them in NATO. And along comes this opportunity, and by three votes they decline the opportunity to allow us to come in through the north."

Northern Iraq is critical to the U.S. plan because it contains major oil fields. It also is home to a sizable Kurdish population that already has a substantial measure of local autonomy and has clashed with neighboring Turkey for years. U.S. authorities want to ensure the Kurds stay largely out of the fight and remain a part of any new postwar Iraq.

If the 4th Infantry Division were redirected to launch from Kuwait instead of Turkey, the cargo ships now in the Mediterranean would require about a week to sail to the Persian Gulf, officials said. But there is some concern that such rerouting could interfere with passage of other military cargo vessels through the Suez Canal -- notably ships with equipment and supplies for the 101st Airborne Division -- and then overwhelm port facilities in Kuwait, which already is hosting several Army and Marine divisions.

Reaching northern Iraq from Kuwait would take an armored force days of hard travel over desert terrain and present a predictable and exposed line of advance. While airlifting a lighter infantry force into the north would provide some U.S. military presence at the outset, the troops could find themselves stretched thin by the multiple challenges of overcoming any Iraqi resistance, securing the region's oil fields and holding Kurdish factions in check.

Being blocked from sending a large armored force into Iraq from Turkey "will not fundamentally affect our ability to succeed militarily, but it will alter our ability to be, in effect, interspersed and be the interlocutors between the Kurds and the Turks," Sen. Joseph R. Biden Jr. (Del.), the top Democrat on the Foreign Relations Committee, said on "Fox News Sunday."

Retired Air Force Gen. Joseph W. Ralston, who stepped down earlier this year as NATO's top commander, told CBS's "Face the Nation" that "instability in the north is something that is very bad for Turkey, and I believe the best way to keep the instability from occurring is to have U.S. forces on the ground in northern Iraq."

Ralston said that although Turkey has been a "good friend" to the United States and a reliable NATO ally in the past, it now has "a very inexperienced government" that was elected in November. "I do believe that we've got to make sure that Turkey understands that this is more in their interest than it is the U.S.," he said.

Gen. Tommy Franks reportedly has not ruled out use of Turkish bases.