The U.N. envoy to Yemen is warning that "the streets are heating up" in the country's restive south, and resentment over more than two decades of unaddressed grievances is reaching "a tipping point."

Calls for an autonomous state in the south — which was once a separate country — have compounded Yemen's problems as the impoverished nation struggles with a resurgent al-Qaida and a deep political crisis.

Jamal Benomar told the U.N. Security Council on Tuesday that a "national dialogue" aimed at mapping out Yemen's future must settle the status of the south in order to develop the foundations of a new constitution.

He urged southern leaders who are not participating to join the talks.

Benomar says demonstrations have increased significantly in frequency and numbers since February.