Northern Ireland, Peter Robinson, Kevin McGuigan, IRA

Northern Ireland First Minister Peter Robinson said Thursday that he is stepping aside because of an impasse that threatens to bring down the Catholic-Protestant power-sharing government.

Robinson said Thursday that all but one of his party's ministers will step down immediately, gravely weakening the coalition government that is a cornerstone of the 1998 peace agreement.

The crisis has intensified with police suggestions that Irish Republic Army dissidents were involved in last month's killing of former IRA member Kevin McGuigan.

Robinson said this made "business as usual" impossible because of "the assessment of the chief constable of the involvement of the IRA in murder, the continued existence of IRA structures, and the arrests that followed has pushed devolution to the brink."

He said he was leaving Arlene Foster in place as a temporary first minister and finance minister to prevent other parties from taking advantage of the situation.

Robinson said crisis talks with the government will continue. The coalition government is on the brink of collapse, which could bring back direct British control from Westminster.

Downing Street officials said Prime Minister David Cameron is "gravely concerned" by the developments and that he will seek a solution.

Sinn Fein President Gerry Adams called for more talks so that the government could continue to operate.

He spoke before Robinson stepped down, but after several parties including Sinn Fein had voted against Robinson's request that the government be suspended to give delicate negotiations a chance to bear fruit.

Several prominent figures have been arrested in recent days, including prominent Sinn Fein regional chairman Bobby Storey. They remain in custody but have not been charged.

The killing was a suspected revenge attack for the killing of a former IRA commander several months earlier.

Critics say the killings show the IRA remains an active and violent paramilitary force despite the peace agreement.