KNIN, Croatia (AP) _ Rebel Serbs in Croatia, recently stung by their biggest losses since the 1991 war, are receiving key help from Yugoslavia to prepare for war, according to U.N. and Serb sources.

The rebel Serbs have lived largely undisturbed since taking a third of Croatia in 1991. But since May they have lost a large chunk of land, and found their stronghold of Knin within range of Croat artillery for the first time.

Now, the army of Serb-dominated Yugoslavia is helping reorganize the rebel forces. Special troops have arrived from Yugoslavia, U.N. sources say, and Serbia is sending back refugees to swell the rebel ranks.

``We are reorganizing our army and in any future attack we will respond professionally and in a different way than before,'' said Col. Kosta Novakovic, assistant commander of the estimated 50,000 rebel Serb troops.

Novakovic said the overhaul is orchestrated by the rebels' newly appointed commander, Gen. Mile Mrksic. A former commander of Yugoslav army special units, Mrksic took over after the Serbs lost the western Slavonia region to Croatia May 1 and 2.

A general mobilization is under way in Serb-held Croatia. Serbia, the dominant republic in what's left of Yugoslavia, has sent back thousands of men who fled during the 1991 war.

Such open support shows that although Serbian President Slobodan Milosevic is playing peacemaker in an attempt to end international sanctions on Yugoslavia, he also is heeding Serbian nationalist and Yugoslav army concerns.

Knin officials insist newcomers from Serbia are all volunteers.

``We have several thousand volunteers from Serbia, and new ones are coming,'' said Ilija Prijic, a close aide to rebel Serb leader Milan Martic. ``And the general mobilization here proved very successful.''

Asked about reports of forced drafting and roundups in Serbia, Prijic said: ``It's high time that those who fled returned.''

Prospects for all-out Croat-Serb war have risen since May. The Serbs avenged the loss of western Slavonia by shelling Zagreb, the capital of Croatia.

Croat and allied Bosnian Croat forces then began creeping up the Dinara mountain range on the Croatian-Bosnian border, and now control peaks that enable them to shell Knin and the rebels' key supply road to Bosnia.

Croatia's President Franjo Tudjman has promised more blitzes if peace talks yield nothing by late fall.

On Sunday, a senior Croat official, Hrvoje Sarinic, charged there were 4,000 Yugoslav soldiers in Serb-held eastern Croatia, two Yugoslav army tank companies in Slunj, south of Zagreb, and two new helicopter rocket launchers at a Serb airfield in southern Croatia.

U.N. sources in Zagreb, speaking on condition of anonymity, confirm a smaller number of Serb soldiers in eastern Croatia and a company of Yugoslav army special forces facing Croats in the Dinara range.

``If it comes to war, it would be a catastrophe for both Croats and us,'' Col. Novakovic said.

The rebel Serbs know the Croatian army is much better armed and trained than in 1991. In addition, the rebels' forced recruits seem to lack motivation.

One of them, who gave his name only as Mirko, lived most of his life in Zagreb and fled to Serbia when the war started. He began a private business in Belgrade and said he never intended to go back to Croatia.

Two weeks ago, special Serbian police knocked at his Belgrade home.

``You don't mess around with special Serbian police. They come and pick you up. You don't have any choice whatsoever,'' he said.