UNITED NATIONS — With plans to strengthen the mandate of the new U.N. envoy to Afghanistan, the United Nations this week is reporting a significant uptick in violence in the country, as Western sources say a resurgent Taliban is receiving support from neighboring Iran.

The Afghan insurgency “has proven to be more resilient than we expected and more ruthless than we ever imagined,” the U.N. undersecretary-general for peacekeeping, Jean-Marie Guéhenno, told the U.N. Security Council yesterday. He spoke as the new U.N. envoy to Kabul, Kai Eide of Norway, was meeting council members for the first time since his appointment last week by Secretary-General Ban. Earlier this week, Mr. Ban reported that violence in Afghanistan rose last year to its highest level since the 2001 American-led invasion that ousted the Taliban from power.

Yesterday, military efforts against the Taliban, which is seeking to regain control over the country, were marred when two women and two children were killed in an exchange of fire between insurgents and NATO forces in the south of Helmand province. In a statement, the British Ministry of Defense took responsibility for the accidental deaths, expressing its “deep regret” and promising an investigation.

“There are very worrying signs of Iranian support for the Taliban on the ground,” a senior Western European diplomat said yesterday, briefing reporters on condition of anonymity. Last June, American officials said they intercepted an Iranian delivery of heavy arms, C4 explosives, and advanced roadside bombs intended for the Taliban to use against NATO forces in Afghanistan.

Western reports of Iranian cooperation with Afghan insurgents have been received with skepticism, as the Sunni Taliban — as well as its Al Qaeda backers — holds Shiite Islam, which is dominant in Iran, in deep contempt. But the director of Israel Radio’s Farsi service, Menashe Amir, dismissed that notion, saying Tehran “will support any force that could help it promote the goal of Islamic victory over the West.” Although there is not as much “flexibility” on the Sunni side on cooperating with the Shiites, he said, the isolated Taliban is known to be desperate and would take support from anyone who offers it.

With several allies withdrawing troops, President Bush on Tuesday called for more Western support for Afghanistan. America will raise its troop level in the country to more than 30,000, half of them in the 40,000-strong NATO force. “We’re spending a huge amount of money on the reconstruction effort,” the American ambassador to the United Nations, Zalmay Khalilzad, told reporters yesterday. “But this is truly an international effort.”

The new U.N. envoy, Mr. Eide, “needs to be empowered” to be able to coordinate with outside donors, the various international actors in the country, and President Karzai’s government, Mr. Karzai said.

Mr. Eide was appointed after Mr. Karzai rejected a previous candidate, Paddy Ashdown of Britain. According to news reports at the time, the Afghan government was uncomfortable with the idea of a “super-envoy” who would have too much power in state affairs. Although a proposed resolution circulated among Security Council members yesterday details similar powers for the new envoy, Mr. Karzai has accepted Mr. Eide’s nomination. Mr. Eide said his policy would be based on an “Afghanization” of international efforts.