The United Nations sees grounds for optimism in Afghanistan despite major problems, a senior UN official said Tuesday at the start of a high-level conference in Geneva that is also looking at the country's security and food crises.

"This is one of the worst moments for the people of Afghanistan, but on many fronts important progress has been made in the last 24 months," said Toby Lanzer, the deputy chief of the UN Assistance Mission in Afghanistan (UNAMA).

Out of a population of 35 million, 3.6 million are threatened by hunger, and half a million have been displaced by violence and drought this year, Lanzer told a press conference.

The UN official stressed, however, that the government has made great strides since the 2016 international donor conference in Brussels.

Kabul had invested in basic supplies for their citizens, strengthened the fight against corruption and reformed security institutions, he said.

"On the reform agenda, there is quicker and better progress than expected," Lanzer said.

An expatriate civil society activist took a starkly different view.

The security situation had worsened in the past two years because of the radical Islamist Taliban rebels and the terrorist Islamic State group, said Jawad Nader, who heads the British and Irish Agencies Afghanistan Group.

"Afghanistan is at a breaking point," he said.

The UN conference is organized by the United Nations and the Afghan government, which is seeking financial and political support from abroad in Geneva, in the face of Taliban attacks and mounting hunger at home.

On the first day of the two-day event, Afghanistan's President Ashraf Ghani is scheduled to lead a discussion on boosting investment in the private sector, especially in the commodities, energy, building and infrastructure sectors.

UN refugee chief Filippo Grandi is set to moderate an event focusing on Afghanistan's migration crisis - one of the world's largest. There are 2.4 million registered Afghan refugees living abroad, as well as an estimated 5 million undocumented Afghans in neighbouring Iran and Pakistan.

Some 20 foreign ministers - including Sergei Lavrov of Russia and Heiko Maas of Germany - are expected to attend the conference.

Taliban militants have ramped up attacks on Afghan security forces and government facilities in recent months, leaving troops thinly stretched. The government has struggled to keep control in many areas since the end of NATO's combat mission there in 2014.

A drought also hit vast parts of the country this year, forcing more than 200,000 people to flee their homes.