An American doctor who contracted Ebola in the Democratic Republic of the Congo has been flown to Germany for treatment, along with his wife and four children, as the World Health Organization warned of the “scale and speed” of the outbreak.

Authorities have reported at least 134 suspected deaths and more than 500 cases of the hemorrhagic Bundibugyo virus, which has no approved treatments or vaccines. The outbreak, which has spread into urban areas, has been declared a public health emergency requiring international response.

Dr Peter Stafford, a surgeon and leader of the Christian missionary group Serge, has said he unknowingly operated on a patient with Ebola before the outbreak was detected. His wife, Rebekah Stafford, also a doctor, and their children, are being monitored for symptoms of the disease.

The infected surgeon was barely able to stand on his own when he departed for Germany, according to two leaders of the Christian missionary group where he worked.

Dr Scott Myhre, area director for Serge told NBC News that Stafford “looked really tired and really sick” as he left. “There were people in full – we call it PPE – the personal protective equipment, and they’re completely covered, and he’s hanging on them barely strong enough to walk.”

Stafford worked at Nyankunde hospital in the DRC’s Ituri province, where the Africa Centers for Disease Control first confirmed the Ebola outbreak. He had operated on a 33-year-old patient with severe abdominal pain.

Doctors at first believed the patient had a gallbladder infection but, according to Myhre, Stafford “did an abdominal procedure and found that the gallbladder was normal and closed him up, but this patient subsequently died the next day”.

The patient was buried before he could be tested for Ebola, but Stafford developed symptoms and tested positive on Sunday, according to the US Centers for Disease Control and Prevention.

Myhre described Stafford as “a very meticulous professional, and for every surgical case he does, he would be completely gowned in sterile garb and gloves and hats and glasses. But that’s not quite enough to prevent an Ebola exposure.”

In an updated advisory on Wednesday, the WHO said there were now more than 600 suspected cases and 139 suspected deaths from the virus, mostly in the DRC.

But with two cases and one suspected death in neighboring Uganda, the organization said the risk of a global pandemic was very low, but the threat for countries in the region was severe.

“We expect those numbers to keep increasing,” Tedros Adhanom Ghebreyesus, WHO’s director-general, said. “We know that the scale of the epidemic in DRC is much larger”.

The director-general also responded to criticism from the US secretary of state, Marco Rubio, of the organization’s Ebola response. Rubio said on Tuesday that the World Health Organization’s response was “a little late”.

“The lead is obviously going to be Centers for Disease Control and the World Health Organization, which was a little late to identify this thing unfortunately,” Rubio said.

But the WHO chief hit back, saying in Geneva that “maybe what the secretary said … could be from lack of understanding of how IHR [International Health Regulations] work, and the responsibilities of WHO and other entities.”