Staff from two aid agencies have been attacked in the last week in Papua New Guinea as violence and tribal conflict hampers the efforts of relief organisations assisting in earthquake recovery.

A team of local aid workers with the PNG Red Cross was robbed and assaulted by armed assailants in Tari. According to police and incident reports seen by the Guardian, the group of eight people, including five PNG Red Cross staff members, was robbed by five people armed with shotguns and machetes last Friday. They had been traveling to Tari, through Hela province, an area which has seen an escalation in violence in recent weeks.

The assailants – who appeared agitated and would not listen to explanations of the Red Cross’s role – allegedly assaulted them and robbed them of clothing and possessions. One man was hit with a machete.

The following day a team of Unicef workers was attacked in the Southern Highlands province, with at least one person believed to have sustained injuries. The team had been driving back to the town of Mendi, said Unicef representative in PNG, David Mcloughlin.

“Our convoy of cars ... was attacked. The worst affected in the attack was a government security escort car. One Unicef staff, a medical doctor, received minor head and shoulder injuries from rocks thrown at the car and with the remaining staff all made it back safely to Mendi traumatised but physical unhurt.

“The injured staff member requiring stitches to their head was treated at a hospital in Mendi. The government and community leaders have apologised to Unicef for the attack.

“We want to reiterate that Unicef appreciates very much the excellent cooperation and support from national and provincial government. We know the marauders do not represent the PNG people who have been kind, helpful and grateful for our assistance.”

Mcloughlin said after the violence began staff were transferred staff to a safe location in Mendi on 29 March. “Although tension in Tari has hindered our humanitarian response, we want to reaffirm that Unicef continues to remain present in the highlands region to provide assistance to needy children. Our team remains in Mendi, despite the car attack, and we are operating from there. For Tari, we are currently reassessing the situation very closely and ready to resume relief work as soon as the security situation allows,” he said.

“The situation is unpredictable and volatile. Security remains a big concern and is affecting our aid delivery. However, Unicef will remain in the highlands region and is committed to providing clean water, hygiene education, vaccination and malnutrition treatment and psycho-social support to children affected by the earthquakes.”

Aid workers have said there is a rising sense of frustration among Highlands residents in some areas, about the level of humanitarian assistance reaching them after the earthquake seven weeks ago – and several subsequent tremors – killed more than 150 people.

More than 20,000 people are estimated to be living in informal relief centres and many affected areas remain cut off by road. More than 140,000 people from the largely subsistence-living population are considered to be food insecure because of the mass destruction of gardens and crops.

The Highlands region is frequently beset by tribal conflicts which have worsened in intensity, and the increasingly indiscriminate targeting of victims. A proliferation of high powered weaponry has left the already resourced-starved police force unable to respond in many cases.

Tari hospital and airport have been closed because of recent fighting during which a number of people were murdered and houses torched.

Last week the UN pulled 12 non-essential staff members from Tari in response to the unstable security situation.

Mark Kessler, head of mission for the International Committee of the Red Cross (ICRC) in PNG said they have not pulled back from their outreach work, but there were growing frustrations and not every NGO had the same in-country recognition and experience as the Red Cross and ICRC.

Kessler said the frustrations were justified in some cases but not necessarily in others.

“The actors, whoever they are, have to be transparent on the services they can provide and when they’ll provide them,” he said.

“If they can’t provide services everyone has to be honest with the population and tell them why.

“There should [also] be a certain degree of patience, understanding and respect at all times towards humanitarian actors and government actors who are doing their best to respond in a complex environment.”