(CNN) -- Ivory Coast President Laurent Gbagbo on Saturday ordered all U.N. peacekeeping forces out of the country a day after Secretary-General Ban Ki-Moon called on the disputed re-election winner to step down.

Gbagbo's reaction to Ban's statement coincides with an increase in pressure Western governments are imposing on the Gbagbo government and a report of an overnight attack on a U.N. peacekeeping patrol.

European Union leaders late Friday threatened sanctions against the west African nation, and the United States announced it would impose "targeted sanctions" against Gbagbo, his family and associates, if the incumbent president does not leave office.

Meanwhile, six armed men wearing military uniforms and traveling in a civilian vehicle opened fire on U.N. peacekeepers in Sebroko overnight Friday, according to a statement Saturday on the peacekeeping operation's website. The U.N. troops returned fire. There were no reports of injuries.

The Gbagbo government accuses the U.N. of providing military and logistical support to the ex-rebels, backers of Alassane Ouattara, the internationally supported winner of the November runoff.

In a Friday national television appearance, Ivory Coast Army Col. Gohourou Babri accused the U.N. of transporting armed supporters of Ouattara to various sites around the country to launch attacks.

Babri said four ex-rebels were killed Saturday in an attack by government forces on a camp in Trebissou near the ceasefire line.

His claims could not be immediately verified.

Violence has also broken out between supporters of Gbagbo and Ouattara. Ban said Friday that Gbagbo's efforts to maintain his office "cannot be allowed to stand."

Ban called Friday for Gbagbo to step down and said that "any other outcome would make a mockery of democracy."

The United Nations, the Economic Community of West African States, the European Union and the African Union have all declared Ouattara as the winner.

"That is our message, as well: The results of the election are known. There was a clear winner. There is no other option," Ban said.

"I call on (Gbagbo) to step down and allow his elected successor to assume office without further hindrance. The international community must send this message, loud and clear. Any other outcome would make a mockery of democracy and the rule of law."

The country's Independent Electoral Commission said Ouattara won the November 28 presidential runoff, but the country's Constitutional Council invalidated those results and declared Gbagbo the winner.

Since then, there has been political stalemate.

At least nine unarmed protesters in Abidjan, Ivory Coast's largest city, were shot and killed by security forces during demonstrations Thursday, witnesses told Amnesty International. The violence erupted as troops loyal to the incumbent president and supporters of his challenger confronted each other.

Saying the "situation has taken a dangerous turn," Ban urged both sides "to avoid provocations or a further escalation of violence."

"Let me say clearly and directly: Any attempt to obstruct U.N. operations or blockade the Golf Hotel is totally unacceptable," said Ban, referring to the site of Ouattara's headquarters.

"Any attack on U.N. forces will be an attack on the international community. I emphasize: Those responsible for the loss of civilian lives will be held accountable."

Journalist Eric Agnero contributed to this report.