UNITED NATIONS/BAMAKO (Reuters) - Dutch forces are preparing to withdraw seven "essential" helicopters from the U.N. peacekeeping mission in Mali, with no clarity yet on how they will be replaced, the United Nations said on Thursday.

"The Netherlands has indicated its intention to withdraw its attack and military utility helicopter units," U.N. peacekeeping spokesman Nick Birnback said.

"The four attack helicopters are essential for the mission, to deter and to respond to attacks," he said, adding that three utility helicopters were also being taken off the mission.

He said the U.N. was consulting with other countries to determine what options may be available. 

The withdrawal, which officials said was owing to their equipment being overstretched, comes as insecurity is worsening in northern Mali. Islamist groups are increasingly staging attacks against the more than 11,000 peacekeepers there, as well as civilians and Malian government officials. The United Nations plans to increase the mission by 2,500 peacekeepers.

Two Dutch peacekeepers were killed by an accidental mortar explosion during a training exercise last week, the latest casualties in the world's deadliest place for peacekeepers to serve.

There are around 400 Dutch troops serving in Mali.

French forces intervened in 2013 to drive back Islamist fighters who had hijacked a Tuareg uprising to seize Mali's desert north in 2012, but Islamist attacks continue, including one that killed two peacekeepers at the end of last month.

Besides Islamist attacks, social unrest linked to dissatisfaction with a peace deal signed a year ago is also undermining peacekeeping efforts. Protests turned violent this week when the military opened fire on a crowd.

Mali's President Ibrahim Keita "expressed deep regret" in a special televised address on Thursday for the killing of three youths by soldiers at that protest, and for other violence dogging Mali.

"The incidents are regrettable and could have been avoided," he said. "I call on all the members of our society to remain calm and cultivate dialogue as .... as way out of this crisis."

(Reporting by Thomas Escritt in Amsterdam, Michelle Nichols at the United Nations and Adama Diarra in Bamako; Writing by Tim Cocks; editing by Grant McCool)