The U.N. special envoy for Burundi urged the African nation's government Thursday to quickly restart dialogue with the opposition following a referendum that will allow the president to stay in power until 2034.

Michel Kafando told the Security Council that Burundi can only solve the current political crisis through "an inclusive dialogue." If this doesn't happen, he warned, the opposition's challenge to the referendum's results "will further polarize an already tense political situation characterized by human rights violations and other abuses as well as the deterioration of its socio-economic and humanitarian situation."

Burundi's main opposition coalition asked the country's constitutional court earlier Thursday to invalidate the results of last week's referendum. The election commission on Monday said over 73 percent of the 4.7 million votes cast supported constitutional amendments, including one allowing President Pierre Nkurunziza to remain in power for 14 years after his current term ends in 2020.

The opposition coalition said the referendum was marred by intimidation and abuses. Before the vote, observers expressed alarm at reported violence and intimidation of the government's perceived opponents, including threats of drowning and castration. A presidential decree criminalized calls to abstain from voting, with a penalty of up to three years in jail.

Kafando drew the Security Council's attention to Burundi's expulsion of experts from the U.N. Human Rights Council on April 26. Their visas were canceled while in Burundi and the U.N. envoy called on the government to facilitate their return. The government also suspended BBC and Voice of America broadcasts.

The constitutional court validated Nkurunziza's third term in 2015, which the opposition and many international observers saw as unconstitutional. Months of deadly political turmoil followed, with more than 1,200 people killed and hundreds of thousands fleeing the country.

Kafando said the security situation is now "generally calm" though there are isolated grenade attacks and reports of bodies being discovered, some in military uniforms. He said the killing of 26 people, including women and children, on May 11 by unidentified armed men in Buganda showed that "the environment continues to be volatile."

The council meeting demonstrated a sharp divide among its five veto-wielding permanent members over the referendum. France, Britain and the United States protested human rights violations and the government's crackdown on the opposition and strongly backed Kafando's call for an inclusive dialogue. Russia and China supported Burundi's government.

French Ambassador Francois Delattre said the constitution's revision goes against the Arusha accords adopted in 2000 to end a 13-year civil war in Burundi that killed about 300,000 people. He warned that mechanisms to protect the Tutsi minority in the Hutu-majority country "are weakened and will even disappear."

Delattre said there has been no success in efforts to start a dialogue between the government and opposition, a push being made by the East African Community, which comprises six countries in the Great Lakes region — Burundi, Kenya, Rwanda, South Sudan, Tanzania and Uganda.

"Nonetheless, the priority should remain a genuine, inclusive national dialogue without any conditions," the ambassador said.

U.S. deputy political coordinator Elaine French echoed the call for talks with the opposition, saying the referendum further aggravates "acute political tensions." Its results "leave us concerned that Burundi is moving closer to one-party rule," she said.

Britain's political coordinator, Stephen Hickey, also urged Burundi's government to pursue an inclusive dialogue and preserve the Arusha accords. "The risks in Burundi are huge and the case for preventive diplomacy is overwhelming," he said.

On the other side, Deputy Russian Ambassador Dmitry Polyansky called the referendum "strictly a domestic affair for Burundi." He said the situation in the country doesn't threaten regional or international peace and security and should therefore be taken off the council's agenda.

Chinese Counselor Zhang Dianbin stressed Burundi's sovereignty and said the referendum "fully proves the Burundi government and peoples' ability to properly address relevant issues" and handle its own affairs.

Burundi's U.N. ambassador, Albert Shingiro, called the referendum "a victim of its success" and sharply criticized unnamed countries that he said "propagated alarmist scenarios" before the vote.

He insisted the new constitution fully respects the Arusha agreement, "to which the government attaches great importance," and guarantees democratic principles, national unity, protection of minorities, ethnic and gender quotas in institutions, and protection of human rights.