( CNN ) -- The Sri Lankan army launched an operation against Tamil rebels in the country 's north early Monday , the military said , claiming to have rescued thousands of civilians trapped in a government safe zone . The Sri Lankan army has relaunched its attacks on Tamil rebels in the country 's north . A rebel Web site , TamilNet , said government forces were engaged in a fresh ground offensive . A TamilNet correspondent in Vanni reported heavy shelling , rocket fire and gunfire . Thirty civilians died in shelling Sunday , rebels said . The government of Sri Lanka has been battling the Liberation Tigers of Tamil Elam ( LTTE ) rebels in a civil conflict that has lasted nearly 25 years . `` Thousands of civilians who had been forcibly held by the LTTE terrorists in the government declared No Fire Zone ( NFZ ) were rescued early hours this morning , 20 April , as the troops engaged in a massive scale rescue mission , were able to open a safe passage for the civilians , '' the Ministry of Defense Web site said . More than 10,000 civilians are trying to seek refuge with soldiers , the military said . The upsurge in hostilities follows a two-day cessation last week for the Sinhala and Hindu New Year . The U.S. State Department on Thursday called for a cease-fire between the Sri Lankan government and Tamil Tigers to allow civilians to escape the fighting . `` We call upon the government and military of Sri Lanka , and the Tamil Tigers , to immediately stop hostilities until the more than 140,000 civilians in the conflict area are safely out , '' acting spokesman Robert Wood said in a statement . `` Both sides must immediately return to a humanitarian pause and both must respect the right of free movement of those civilian men , women and children trapped by the fighting . '' Watch the heavy toll of fighting on civilians » . A brief cessation of hostilities announced by the Sri Lankan government on April 12 allowed the United Nations and its partners to bring in aid , but a renewed government offensive has left civilians trapped in a war zone . U.N. Secretary-General Ban Ki-moon and his humanitarian chief John Holmes called the brief halt in fighting inadequate and have pushed for a longer humanitarian pause in fighting . The most pressing concern now , according to Holmes , is the fact that more than 100,000 people are crowded in a `` very small pocket of land '' that is about five square miles , or about twice the size of New York 's Central Park . `` It is a very small area indeed for what we believe is a very large number of people , '' Holmes told reporters Wednesday . Overcrowding is also a problem in camps housing displaced people , according to the United Nations Children 's Fund ( UNICEF ) . As of Monday , some 65,000 displaced people were crammed into camps in Vavuniya , Jaffna and Mannar , with 35,000 more expected to arrive within 48 hours , the organization said in a statement . ' UNICEF said it feared for children trapped in the escalating fighting and is worried that the worst is yet to come . The group appealed for donations to help cover `` the most immediate needs of the affected population in health and nutrition , water and sanitation , protection and education . '' The crowded population , primarily comprised of Tamil civilians , is subject to the continuing Sri Lankan government assaults . The British and French foreign ministers released a joint statement as well on Wednesday , saying that the Tamil Tigers are using Tamil civilians as human shields . The Tigers have been declared a terrorist organization by 32 countries , including the United States and the European Union . As many as 70,000 people have been killed since the conflict officially began in 1983 . The International Committee of the Red Cross said Monday that it has helped evacuate more than 10,000 sick and injured patients , and their caregivers , from Putumattalan in rebel-held territory since February . `` These evacuations have saved many lives , '' said Morven Murchinson , the Red Cross medical coordinator in Sri Lanka . `` It is vital that they continue , because more sick and wounded people are arriving every day at the makeshift medical facilities in Putumattalan . '' The Red Cross says there 's an acute shortage of medical supplies in the region , which it is trying to rectify .
NEW : Fears voiced by UNICEF for children trapped in the escalating fighting . Upsurge in hostilities follows cessation for New Year . U.N. humanitarian chief : Large number of people crowded in very small area . Tamil Tigers , Sri Lanka government locked in conflict lasting nearly 25 years .