Islamabad, Pakistan - The United Nations has thrown its weight behind a peace process aimed at ending almost 20 years of devastating war in Afghanistan, with Secretary-General Antonio Guterres calling it an opportunity that "cannot" be missed.

Guterres was speaking at a high-level international summit in Pakistan's capital, Islamabad, held on Monday to mark 40 years of Afghanistan's southeastern neighbour hosting millions of refugees from armed conflict.

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In his speech, the UN chief urged the international community to aid Afghan refugees who reside in Pakistan and Iran and "do everything possible" to achieve peace in Afghanistan, where tens of thousands of civilians have been killed since a United States invasion in 2001.

"We do not have the right to miss this opportunity," said Guterres, who is in Pakistan on a four-day trip. "No Afghan will forgive us if this opportunity is missed."

His comments came as the US and the Taliban appear to be close to embarking on a seven-day "reduction in violence" in Afghanistan as a prelude to a peace deal to the US's longest war. The two sides have been wrangling over Washington's demand for a ceasefire before the signing of a final peace agreement, which is expected to outline the withdrawal of US troops and a guarantee Afghanistan will not be used as a launchpad to conduct attacks abroad.

The deal also proposes talks between the government in Kabul and the Taliban. The armed group, which has been fighting the US-led NATO forces in Afghanistan since it was toppled from power in 2001, has so far refused to speak to the Western-backed Afghan government, calling it a "puppet regime".

In October, the UN Assistance Mission to Afghanistan (UNAMA) said it chronicled record-high civilian casualties from the war with the Afghan Taliban. In total, at least 2,563 civilians were killed and more than 5,600 wounded in the first nine months of 2019, UN data showed. UNAMA has recorded at least 27,390 confirmed civilian casualties as a result of the war since 2009, when it began gathering data under a standardised methodology.

Khan: Pakistan supports peace process

The rocky peace process looked to have hit an impasse in September last year, when US President Donald Trump decided at the 11th hour to call off planned talks with Taliban leaders at his presidential Camp David retreat.

Since then, the US's Afghan peace envoy Zalmay Khalilzad has been rebuilding the process, holding talks with Taliban officials in Qatar's capital, Doha, as well as with Afghan government leaders in Kabul and Pakistani military and civilian leaders in Islamabad.

On Thursday, Trump said a peace agreement was "very close" and could be signed in the next two weeks.

Speaking at the same event as Guterres, Pakistani Prime Minister Imran Khan reiterated the resolve of his government - accused for years by the US and Afghanistan of supporting the Taliban - to back the peace process.

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"Our security forces are on the same page," he said on Monday. "There was an idea that the security forces in Pakistan had their own policy and the government had their own policy. This is not the case any more."

Khan said Pakistan had facilitated the US-brokered peace process, and that "it is not in the interest of Pakistan for there to be any strife in Afghanistan".

More funding needed

Pakistan is home to at least 1.4 million registered refugees from Afghanistan, per data from the UN High Commissioner for Refugees (UNHCR). At least as many are estimated by the authorities to be living in the country without official refugee cards, which Pakistan stopped issuing to new refugees in 2015.

Monday's summit was attended by the UN chief, Pakistani prime minister and other top officials from Afghanistan and Iran, another Afghan refugee-hosting country. Participants called on the international community to increase funding to help the millions of Afghans who remain displaced by the 19-year war between the US-led NATO forces and the Afghan Taliban.

"For 40 years, the people of Afghanistan have faced successive crises. For 40 years, the people of Pakistan have responded with generosity," said Guterres, who previously served for 10 years as UNHCR chief. "We must recognise that international support for Pakistan has been minimal compared to [Pakistan's own funding]. As we look to the challenges ahead, the global community must step up."

UNHCR's budget for operations in Pakistan is $35m, roughly 21 percent of which is currently funded, according to UNHCR data. Pakistan allocated $3.5m of its federal budget this year for Afghan refugees.

For his part, Khan said he was disappointed by how European countries and others had treated arriving Afghan refugees.

"Political leaders use refugee problems to divide humanity. To get votes, they transfer hatred towards refugees who are already suffering ... They face xenophobia in so many affluent, rich countries."

Pakistani and UN officials encouraged the voluntary repatriation of Afghan refugees to their home country through a UN-assisted system. An increase in violence in Afghanistan has seen a dramatic drop in such returns in the last year.

In 2019, at least 8,079 refugees returned home to Afghanistan, including 6,062 from Pakistan and 1,939 from Iran, according to the UN data. That number was 59 percent lower than voluntary returns in 2018, and the lowest since the UN began keeping records following the 2001 outbreak of the war between the US-led NATO forces and the Taliban.

"From the refugee perspective, that path remains uncertain," said Filippo Grandi, the UNHCR chief. "Inside Afghanistan fighting continues to kill and maim civilians, shut down schools and clinics and limit economic activity.

"More than 400,000 people were displaced within the country last year alone."

Asad Hashim is Al Jazeera's digital correspondent in Pakistan. He tweets @AsadHashim