Nigerian President Goodluck Jonathan has declared a state of emergency in parts of the country following attacks from the Islamist group Boko Haram.The measure is in force is areas of the Yobe and Borno states in the north-east, Plateau state in central Nigeria and Niger state in the west.International borders in the affected areas have been temporarily closed.In separate clashes, at least 50 people have been killed in the east of the country over a land dispute.A government spokesman said the violence in the state of Ebonyi was the result of a land dispute between the Ezza and Ezilo peoples.Regional presenceMr Jonathan vowed to "crush" Boko Haram, which killed dozens in attacks across the country on Christmas Day.Announcing the state of emergency in a live televised address, Mr Jonathan said: "The temporary closure of our borders in the affected areas is only an interim measure designed to address the current security challenges."There is growing concern that Boko Haram has developed a presence across the region.Earlier this week, leaders of neighbouring Chad and Cameroon were reported to have held talks about how they can help prevent the violence spreading to their countries.Mr Jonathan's announcement means parts of Nigeria's border with Niger, Chad and Cameroon will be sealed until further notice.The president added that his chief of defence staff had been instructed to take other "appropriate" measures, including setting up a special counter-terrorism force.The BBC's Tom Oladipo in Lagos says Mr Jonathan has been under increasing pressure to address the violence, and this announcement reveals his toughest stance so far.Boko Haram is thought to be responsible for a string of major attacks across Nigeria this year.It bombed the headquarters of the UN in August 2011, leaving at least 21 dead and has targeted police as well. It has also in the past targeted Muslim leaders.At least 42 people died in the Christmas Day bombings of churches and other targets. Tens of thousands of people have fled their homes in Maiduguri in Borno State, as well as other cities.