UNITED NATIONS (Reuters) - The United States, Britain and the United Nations were skeptical on Wednesday of South Sudan President Salva Kiir’s declaration of a unilateral ceasefire, noting that it coincides with the start of the rainy season that traditionally lessens fighting.

Kiir also said on Monday he would release political prisoners, but with no sign of a political deal with rebels it was not clear whether a ceasefire would take hold. [nL8N1IO3NE]

U.N. South Sudan envoy David Shearer welcomed the announcements, but warned they would be closely scrutinized. U.N. peacekeepers have been deployed since South Sudan gained independence from Sudan in 2011.

“The proof of the pudding, as they say, is in the eating,” Shearer told the U.N. Security Council on Wednesday.

South Sudan has been mired in a civil war since 2013, when Kiir fired his deputy. The conflict, fanned by ethnic rivalries, has sparked Africa’s worst refugee crisis since the 1994 Rwandan genocide and plunged parts of the country into famine.

British U.N. Ambassador Matthew Rycroft said the onus was now on Kiir to “prove that these are meaningful commitments, not just convenient timing with the start of the rainy season when fighting is made more difficult.”

“I note with skepticism, but not surprise, that on the day the ceasefire was announced, government offensives continued in Yei and attacks by militia groups continue still,” he said.

Deputy U.S. Ambassador to the United Nations, Michele Sison, said the ceasefire pledge by Kiir came two months after he had promised it to the east African regional bloc IGAD.

She told the council it also only came after government troops pursued “offensives in multiple locations in South Sudan before the onset of the rainy season” and that there had been no end to the obstacles faced by U.N. peacekeepers.

The Security Council has long threatened to impose an arms embargo on South Sudan. But when the United States put the measure to a vote in December it failed to get the nine votes needed to pass. [nL1N1EI0RL]

Both Rycroft and Sison pushed the council on Wednesday to impose further sanctions. Russia against expressed it opposition to an arms embargo, adding that it hoped the ceasefire is implemented.

Kiir on Tuesday announced that Lieutenant General Marial Chanuong - blacklisted by the Security Council in 2015 - would head the government ground forces. Chanuong was sanctioned for leading “the slaughter of Nuer civilians in and around Juba, many who were buried in mass graves,” in 2013.

The United States said on Wednesday it would provide an additional $329 million in humanitarian assistance to help provide food, water, medical care and shelter to people affected by famine and violence in South Sudan, Nigeria, Somalia and Yemen.

With the new funding, the overall U.S. humanitarian assistance for those four crises comes to $1.2 billion since the beginning of the 2017 fiscal year.