South Sudan's government has allocated $40m to speed up the integration of its soldiers with armed rebel fighters in order to create a unified national army - one of the provisions of a stalled peace agreement aimed at creating a power-sharing government after years of civil war.

The move reported on Friday came as President Salva Kiir and rebel leader Riek Machar held rare face-to-face talks in the capital, Juba. The cash injection is part of $100m pledged by Kiir's government to help fund the peace process - a promise yet to be delivered.

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The talks are set to continue through Saturday as the two men try to meet an extended deadline for forming the unity government.

The international community has been pressing the rival sides to form a coalition as a crucial part of the peace deal signed last year. A November 12 deadline to form that government was extended to February at a meeting in Uganda last month.

Kiir said he accepted the extension to avoid the risk of returning to war.

In a sign of growing international pressure, the United States on Thursday threatened visa restrictions on anyone who attempted to damage the peace process.

US Secretary of State Mike Pompeo said Washington would implement visa restrictions barring those who "undermine or impede the peace process in South Sudan" from entering the US, including those who violate a ceasefire or United Nations arms embargo, commit human rights abuses or engage in corruption.

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"The people of South Sudan have suffered enough while their leaders delay the implementation of a sustainable peace. The South Sudanese deserve leaders who are committed to building consensus and willing to compromise for the greater good," Pompeo said in a statement, adding that the visa restrictions could also apply to family members.

The announcement came after Washington earlier this week imposed sanctions on five South Sudanese officials it said were responsible for the likely murders of two human rights activists in 2017.

Civil war broke out in oil-producing South Sudan in 2013 - less than two years after the country gained independence from Sudan following decades of war - after Kiir sacked Machar as vice president accusing him of plotting a coup.

The conflict has killed an estimated 400,000 people and forced roughly four million people - one-third of the population - from their homes.