Violence in Afghanistan increased dramatically in the first four months of this year, the UN says.In a quarterly report to the UN Security Council, it said roadside bomb attacks rose by 94%, compared with the same period in 2009.
On average, the report said, there were three suicide bombings a week, half of them in the country's volatile south.
The findings come amid a major Nato-led operation in Helmand and a surge of US reinforcements.
US President Barack Obama ordered 30,000 extra troops to Afghanistan in December.
Assassinations rise
And Nato launched Operation Moshtarak in Helmand province in February, its biggest military offensive since the Taliban were ousted in 2001.
The report noted the rise in violence was "attributable to an increase in military operations in the southern region during the first quarter of 2010".
It also said Afghanistan's overall security situation "has not improved" since the UN's last report in March.
Despite all this, the electoral commission had successfully registered more than 2,500 political candidates - including 400 women - for polls due in September, the report said.
It also noted that both the Afghan police and army were slightly ahead of interim targets for beefing up their ranks.
But assassinations had risen 45%, with the Taliban and others increasingly successful at killing Afghan officials, the report said.
It added that sophisticated suicide bombings had doubled from last year to roughly two per month.
"The shift to more complex suicide attacks demonstrates a growing capability of the local terrorist networks linked to al-Qaeda," the report said.
It also noted: "The alarming trend of increased improvised explosive device incidents and the occurrence of complex suicide attacks persisted."
'Tough fighting'
UN Secretary General Ban Ki-moon submitted the report to the Security Council this week.
Nato spokesman Brig Gen Josef Blotz told reporters in the Afghan capital Kabul on Saturday: "Tough fighting is expected to continue, but the situation is trending in our favour as more forces flow into the area."
"It has to be tougher, perhaps, before it goes easier," he added, reports the Associated Press news agency.
Gen Blotz also said there had been 44.4% fewer civilian casualties in the last three months, compared with the same period in 2009, due to more stringent rules of engagement.
Operation Moshtarak, involving 15,000 troops in the Marjah area of Helmand, has been hailed by Afghan and Nato officials as a success.
But there have been reports of ongoing violence, Taliban intimidation and a lacklustre performance from Afghan police and civilian administrators, say correspondents.
Nato-led forces are meanwhile poised to launch a long-planned offensive in the southern city of Kandahar, a stronghold of the Taliban.
According to a tally kept by the independent iCasualties website, more than 1,120 US soldiers and almost 300 British soldiers have been killed in Afghanistan since 2001.
President Obama's strategy envisages foreign troop numbers in the country peaking at 150,000 by August, before a US drawdown in 2011.
