(CNN) -- Fresh fighting erupted Sunday between Somalia's transitional government forces and Islamist rebels, continuing a wave of violence that a top United Nations official called a "grave violation of human rights" that could possibly amount to war crimes.

The fighting restarted Sunday with heavy fire in a district north of the capital, Mogadishu, Somalia's Shabelle radio reported, quoting officials and witnesses.

On Saturday, the security director for the transitional government was killed and at least 40 people, mostly civilians were rushed to Mogadishu hospitals, according to Shabelle.

The United Nations' human rights chief said the strife in Somalia has caused "enormous suffering and massive displacement."

"In this new wave of attacks, it is clear that civilians -- especially women and children -- are bearing the brunt of the violence," said Navi Pillay, the U.N. High Commissioner for Human Rights.

She said attacks against civilians have been a signature feature of the conflicts that have plagued the Horn of Africa nation.

"There needs to be a much greater effort to protect civilians. Displaced people and human rights defenders, aid workers and journalists are among those most exposed, and in some cases are being directly targeted," she added.

Somalia has been mired in chaos since 1991, when warlords overthrew dictator Mohamed Siad Barre and sparked brutal clan infighting.

The transitional government has struggled to establish authority, challenged by Islamist groups that have seized control of Mogadishu and much of the south.

The United Nations estimates that more than 200,000 people have been forced to flee Mogadishu since the latest round of fighting began in early May between the government and the Al-Shabab and Hisb-ul-Islam groups.

U.N. investigators, who spoke with refugees and internally displaced persons, reported that militants had carried out executions, planted land mines, bombs and other explosive devices in civilian areas, and used civilians as human shields, according to a statement from the office of the high commissioner for human rights.

The high commissioner also said there were reports of torture and evidence of child recruitment by forces fighting in Somalia.

"Some of these acts might amount to war crimes," Pillay said.