JUBA (Reuters) - Hundreds of protesters massed outside the U.S. embassy and U.N. headquarters in South Sudan’s capital on Tuesday, chanting slogans against an arms embargo imposed by Washington, before some attacked journalists at the scene.

The demonstrators handed in a petition to the United Nations, then some in the crowd turned on reporters, punching them and throwing stones, witnesses said. One foreign journalist needed medical treatment.

“She was targeted because when the demonstrators saw her, they said ‘Why is the white person taking our photos?’ ... they beat her,” said a local journalist who asked not to be named.

The United States banned the export of weapons and defense services to South Sudan on Friday, in a bid to press President Salva Kiir to end a four-year-old civil war.

Washington has already blacklisted several South Sudanese figures - the former army chief and several former or serving officers, a businessman and the information minister - that it says have helped stoke the conflict, which has triggered Africa’s biggest refugee crisis since the Rwandan genocide.

Protesters held banners denouncing colonialism and chanted “America mind your own business, stop the arms embargo against South Sudan”.

“We are ready to defend South Sudan from any foreign invaders and oppression even if it means to die,” read the petition, presented by Gatluak Bol Dhew, the acting secretary general of South Sudan National Youth - a youth group which says it is not linked to the government or any political party.

The petition praised China - which has large investments in South Sudan’s oil industry - and Russia as “true friends” of the country.

More than a third of the population has been driven from their homes by the fighting, which had exacerbated ethnic divisions, splitting the oil-rich nation into a patchwork of fiefdoms.

Internationally-backed attempts to mediate a peace deal or ceasefire have repeatedly failed, and deadly attacks on aid workers trying to tackle widespread hunger, poverty and disease are common.

Police spokesman Daniel Boulogne said they were unaware of any violence at the demonstration.

When asked about the attacks on journalists, he said “this was not reported to us.”

The United Nations Mission in South Sudan said the demonstration was largely peaceful, but added that “a small group of those gathered outside threw stones at the main gate of the base”.