BELFAST, Northern Ireland (AP) _ A Roman Catholic policeman escorting children on an outing to promote sectarian goodwill was ambushed and killed by IRA gunmen, police said.

The victim, Sgt. Frank Murphy, was the second Catholic security officer killed in Northern Ireland this week.

Murphy, 30, worked for the community relations branch of the Royal Ulster Constabulary, Northern Ireland's regular police force. The father of three was one of the few Catholics in the 90 percent Protestant police force, a police spokesman said.

Murphy was bringing children back to Killyleagh from the town of Armagh, where he had organized a quiz for Catholic and Protestant children in an effort to overcome sectarian divisions, the spokesman said.

He had just returned the 14 children to their school, five miles from the border with the Irish Republic, on Thursday when gunmen opened fire from a roadside cottage, the police spokesman said. Murphy died instantly.

The gunmen, who were holding the cottage's residents hostage, then ran out and pumped more bullets into Murphy's body, said the spokesman, who spoke on condition he not be identified.

The Irish Republican Army claimed responsibility for the attack without giving a motive. The killing aroused fears that the IRA, which is almost exclusively Catholic, has renewed a campaign to prevent Catholics from joining the Royal Ulster Constabulary. The IRA seeks to end British rule over Northern Ireland and unite the province with the Irish Republic.

Meanwhile, an official source said intelligence officers in the United States, Britain and the Irish Republic collaborated in tracing an alleged IRA bank account of 1.75 million Irish pounds, the equivalent of $1.64 million, seized by the Irish government.

The source, who spoke on condition he not be identified, told reporters U.S. intelligence officers helped Irish authorities chart the progress of IRA money moving from Britain to Ireland via the United States and Switzerland.

He said the government believed that in seizing the account in a bank near Dublin it had confiscated a ''major portion'' of IRA funds in the Irish Republic used to finance guerrilla action.

He gave no further details on the U.S. and British involvement.

The IRA in statements to the news media Thursday said it had no connection with the money.

The money is believed to be the proceeds of kidnapping and protection rackets and was surrendered by the Bank of Ireland branch in Navan, 40 miles northwest of Dublin, after an official order from Justice Minister Michael Noonan.

The British news agency Press Association reported that U.S. authorities ''are thought to have stumbled on the IRA connection during investigations into a bank in Boston'' over a suspected link there with organized crime.

The cash, now in the hands of the Irish High Court by virtue of emergency legislation passed earlier this week, could be distributed to charities if there is no claim to ownership within six months, Press Association said.