British army reinforcements arrived in Northern Ireland today to combat an expected Christmas season terrorist campaign, the army said. The army headquarters in Northern Ireland said the extra 600 troops were being deployed to help the Royal Ulster Constabulary, the province's police force. It is the first time in four years that reinforcements have been sent to the British-run province, the army said. ``This is not a reactive measure but part of a prudent management of security force activity,'' an army spokesman said. ``They will be retained in Northern Ireland as long as it is absolutely necessary.'' The constabulary said intelligence reports indicated that both the outlawed Irish Republican Army, which is mainly Roman Catholic, and Protestant paramilitary groups planned an increase in attacks in the coming weeks. A spokesman refused to say where the troops will be based, but sources said they would be spread throughout the province, increasing patrols and roadblocks. Seventy-two people have been killed in terrorist incidents in 1990, 27 of them since October. In 1989, 62 people were killed. The reinforcements bring the number of British troops in Northern Ireland to about 11,000, the Ministry of Defense said. Britain sent troops to Northern Ireland when sectarian and political violence erupted in August 1969. Since then, nearly 2,900 people have died in the violence.