The Nato commander in overall charge of Kosovo has likened the recent violence in the province - in which at least 28 people have died - to ethnic cleansing.
Nato troops are out in force in the flashpoint town of Mitrovica
Admiral Gregory Johnson said almost 1,000 Serbs had been driven from their homes after attacks by ethnic Albanians
It is the worst outbreak of violence since Nato forces entered Kosovo in 1999 to end years of ethnic unrest.
Meanwhile, reinforcements for Nato peacekeepers have started arriving in Kosovo following the deadly clashes.
Admiral Johnson also said he believed that some of the trouble had been orchestrated.
He added that Nato peacekeepers were investigating whether the violence had been organised by ethnic Albanian militants.
Serbian Prime Minister Vojislav Kostunica has himself described the attacks as "planned in advance and co-ordinated... this was an attempted pogrom and ethnic cleansing" against Kosovo's Serbs.
Serbia has also has accused both the UN and Nato of failing to protect Kosovo's Serbs.
On Friday UN Secretary General Kofi Annan said the decision to send Nato reinforcements had been taken in view of the worsening security situation since trouble erupted earlier in the week.
UN spokesman Fred Eckhard said that the reinforcement troops' presence "is being felt" and, while there have been reports of more clashes and looting, most incidents have been on a smaller scale compared to earlier in the week.
Fear and apprehension
Nonetheless, BBC correspondent Nick Thorpe says the atmosphere in the flashpoint town of Mitrovica - where violence began on Wednesday - remains one of fear and apprehension.
Groups of Serbs have gathered near each of the three bridges which span the River Ibar which Nato peacekeepers have blocked off.
Their numbers could swell in a matter of moments if there is any sign of attack from the Albanian side, our correspondent says.
However, despite occasional shots and explosions there has been no repeat of the violence, although some Serbs admit to having guns which they say they will use if necessary to defend their part of the city, our correspondent adds.
Initial reports said 31 people had been killed in the two days of clashes.
Trouble first erupted in the divided city of Mitrovica after the deaths by drowning of two Albanian children, which were blamed on members of Kosovo's small Serbian minority.
KOSOVO: KEY DATES
24 Sept 1998: Nato issues ultimatum to Milosevic to stop crackdown on Kosovo Albanians
24 Mar 1999: Nato begins air strikes against Yugoslavia over Kosovo
10 June 1999: Air strikes suspended after Milosevic agrees to withdraw troops. UN approves peace plan for Kosovo, establishes K-for peace force
11 June 1999: Nato troops enter Kosovo
10 Dec 2003: UN unveils road map on conditions Kosovo must meet by mid-2005 for talks on final status
17 Mar 2004: Serbs and Albanians clash in the worst violence seen since 1999
Mobs of angry Albanians set alight Serbian Orthodox churches and Serb-owned homes across Kosovo on Thursday.
On Friday Nato troops in Mitrovica shot and killed a sniper who fired at peacekeepers from a block of flats chiefly housing ethnic Albanians in the northern half of the town, Nato spokesman Lieutenant Colonel Jim Moran said.
Reports quoting Serbian sources said the sniper was ethnic Albanian.
Earlier French forces searched blocks of flats near the river Ibar early in pursuit of gunmen after reports of attacks overnight.
"The situation is calm but very volatile, very fragile and could escalate any minute," one French officer told the Associated Press.
In Serbia itself on Friday thousands of people demonstrated peacefully in central Belgrade following a government call to show solidarity with Serbs living in Kosovo.
Carrying Serbian flags, pictures of Kosovo monasteries and religious icons, the crowds chanted "We're not giving Kosovo away" and "Kosovo is Serbia".
The first extra contingent of 750 peacekeeping soldiers being sent by the UK arrived in the region's capital Pristina overnight.
Germany's Defence Minister Peter Struck said on Friday that a further 600 peacekeepers were being sent to join German forces in Kosovo, with deployment starting on Saturday.
France also says it is sending about 400 more troops immediately. Denmark has pledged 100 more.
About 18,500 peacekeepers are already based in Kosovo.
MITROVICA: DIVIDED TOWN
Pre-war population: 48,500 Albanians, 8,100 Serbs, 11,300 others
Bridge 1 guarded by Serb "bridge-watchers"
K-For troops have usually kept bridge 2 open for Albanians to cross
Explosion reported in apartment block north of footbridge 3