* Impatience growing over peace process delays

* Rebels say U.S. withdraws flight funding

* Chief of staff arrives, no news when Machar will follow (Adds arrival of Machar's chief of staff in Juba)

By Denis Dumo

JUBA, April 25 (Reuters) - Washington accused both sides in South Sudan's two-year conflict of blocking peace efforts and protested, rebels said, by pulling funding for a flight to return their leader Riek Machar to the capital.

Underlining growing international frustration over months of delays and wrangling, the U.S. State Department said South Sudan's government had as recently as Saturday refused to give landing permission to planes carrying Machar.

Machar himself, the United States said, had obstructed arrangements by arbitrarily asking for more forces and heavy weapons to precede his arrival.

Machar's return to join a unity government with his foes, originally scheduled for early last week, was meant to seal a peace deal signed in August to end fighting that has killed thousands and forced a million to flee their homes.

"Given the actions by both sides to prevent or delay his return, it is now time for the parties to assume primary responsibility for facilitating the return of Riek Machar to Juba," the State Department said late on Sunday.

In one possible sign of progress, Machar's chief of staff, General Simon Gatwech Dual, flew into in Juba on Monday, accompanied by the 195 soldiers and the weapons the rebel leader had asked for. But he did not say when Machar would follow.

"NO MORE WAR"

"I am happy that I am in Juba. Our coming is to implement the peace process and we are not going back to war," he told reporters at the airport, but did not take questions.

Washington, which was a major player in the accord that eventually secured South Sudan's secession from Sudan in 2011 and has been a donor ever since, said its future engagement would depend on the leaders' involvement in the peace process.

Kiir's sacking of Machar as his deputy in 2013 triggered fighting between their supporters that spread across the impoverished, oil-producing country, often along ethnic lines between Kiir's dominant Dinka ethnic group and Machar's Nuer.

They signed the peace deal under pressure from the United States and the United Nations, which threatened sanctions. But distrust, exacerbated by past splits during South Sudan's long wars with Sudan, runs deep.

William Ezekiel, spokesman for Machar's SPLM-IO group, said the United States' s decision to withdraw funding for a charter flight would delay the return for yet another day.

"Right now, we are still working on the issue and probably by tomorrow the first vice president will arrive in Juba," he told Reuters.

"If it turns out that the Americans are not ready to facilitate him, nevertheless, the government may take over," government spokesman Michael Makuei said, without going into further details. (Reporting by Denis Dumo; Additional reporting to Terje Solsvik in Oslo; Writing by Elias Biryabarema and George Obulutsa; Editing by Andrew Heavens)