Nikolic warned that Serbia will defend "every inch of its territory", including its former province of Kosovo where ethnic Serbs allegedly are under threat from Kosovar Albanians. "I will be the first one, who will go there, that is not new for me", Nikolic said following the session of the National Security Council answering the question whether Belgrade will send troops to Kosovo.
Kosovo police blocked the entry on Saturday of a train from Serbia painted in the colours of the Serbian flag and with the words "Kosovo is Serbia" printed down the side, in a standoff seen as a test for the tiny Balkan country.
Kosovo declared independence from Serbia in 2008, but while it is supported by many in the West, Serbia does not recognise the split.
Serbia officially is seeking European Union membership, but has lately been sliding toward the Kremlin and its policies to increase its influence in the Balkans.
In reaction to the Saturday incident, Serbian Prime Minister Aleksandar Vucic said he had ordered the train to return to Belgrade from the border because of fears it would come under attack.
But Pristina has denounced it as a "serious provocation" challenging Kosovo's territorial sovereignty. He accused the Kosovo Albanians of "wanting war".
"This was an ambition to provoke a conflict, to start a wider conflict in this territory that we consider as ours", he said.
Serbia retains effective control of the northern part of Kosovo, which is home to 40,000 to 50,000 Serbs who do not see Pristina as their capital.
2008 - Kosovo unilaterally declares independence.
Mr Haradinaj has twice been cleared of such charges at the United Nations, but Serbia insists he should be extradited. North Atlantic Treaty Organisation airstrikes on Serbia forced Belgrade to withdraw its troops in 1999, having killed 10,000 Albanian civilians there.
"Yesterday we were on the verge of clashes", he added.
Kosovo had deployed its special forces to prevent the train from crossing its border, The Associated Press reports.
Serbia denied that the Russian-made train's journey was provocative, despite it coming amid heightened tension due to the detention in France of ex-Kosovo prime minister Ramush Haradinaj on a warrant issued by Belgrade.
Kosovo Albanians make up more than 90 percent of Kosovo's 1.8 million population.