ZAGREB, CROATIA — A DAY after a cease-fire brokered by former President Jimmy Carter expired in Bosnia, the widest-ranging fighting in three years has broken out in the Balkans. Mortars struck the Bosnian capital of Sarajevo on May 2, and rebel Serbs rocketed the Croatian capital of Zagreb, threatening an all-out Balkan war.
''We're clearly at a very difficult and dangerous moment.... The risks of a wider war are real,'' Robert Hunter, United States ambassador to NATO told Reuters.
The United States and its NATO allies stands ready to aid in the withdrawal of about 20,000 United Nations peacekeepers from the region, if necessary. Rebel Serbs were still detaining more than 100 peacekeepers in Croatia at press time, and three Jordanian peacekeepers were seriously wounded May 1 in a Croatian artillery attack.
US Ambassador to Croatia Peter Galbraith told CNN May 2 that the rocket attacks on Zagreb amount to a declaration of ''full-scale war.'' He said the rebel Serb attacks are potentially ''the most serious escalation'' since war began in Croatia in 1991.
The increasingly bold Croatian government pressed on with its high-stakes military offensive May 2 that government officials are publicly describing as ''limited'' and privately hoping will stay under control.
Nearly 3,000 Croatian government troops are attempting to wrest control of a strategic 20-mile stretch of highway from rebel Serbs.
Croatian officials scrambled to play down the operation. ''It is definitely limited in scope, the number of people involved, and time,'' says a senior Croatian diplomat. ''The goal is to gain control of the highway and that's all.''
UN officials reported significant gains by Croat forces pushing from both ends of the highway. ''I would say that they have about [12 miles] of highway to go, and they may be able to secure that [May 2],'' said a UN official. ''We suspect they want to take the highway and then talk from there.''
Whether the operation succeeds may be the clearest sign yet that the tactical balance in the former Yugoslavia is shifting. Observers say the current operation and successful Bosnian government offensives this spring suggest that rearmed and reorganized Croatian and Bosnian government forces are now able to exploit their numerical superiority against well-armed, but thinly spread Serb forces in Croatia and Bosnia.
Croatian and Western observers say time is the key. If Croat forces can quickly take the highway -- punching a large hole in Croatian Serb political and military confidence -- the fighting can be contained. But the longer the operation drags on, the greater the risk of Serbs in neighboring Bosnia and Serbia joining the fray -- sparking the most intense fighting ever in the former Yugoslavia.
The area involved in the fighting is an emotional one for Serbs. Croatian forces captured the town Jasenovac, the site of a Croatian-run concentration camp where tens of thousands of Serbs were killed during World War II. Croatian troops reportedly destroyed a memorial at the site.
Observers fear that television images of victorious Croats, who Serbian nationalists claim are eager to commit new atrocities, may pressure Serbian President Slobodan Milosevic to come to the aid of his fellow Serbs in Croatia. Bosnian Serb leader Radovan Karadzic vowed May 2 to aid Serb forces in Croatia.
Croatian jets continued on May 2 to target the one bridge that would allow Bosnian Serbs to funnel reinforcements to the area, which is considered the most vulnerable of three Serb strongholds in Croatia. Croatian jets also attacked the headquarters of Croatian Serb leader Milan Martic, according to UN officials.
The Croatian attack is an embarrassment for the US, which has been one of the country's strongest backers since it declared independence from the former Yugoslavia in 1991. Croatian officials say the operation is in response to clashes over the last week that left one Serb and five Croats dead along the highway, which was recently reopened under UN administration.
''If this is a police operation to get rid of terrorist elements that's one thing,'' says Slaven Letica, a Croatian political commentator and former national security adviser to President Franjo Tudjman.
''If it's extended to involve broader military activities, it's going to be riskier from a political point of view in terms of the West and the Croatian public. No one is interested in war,'' Mr. Letica adds.