British former Royal Marine Paul "Pen" Farthing returned to the UK on a privately funded charter flight to London's Heathrow Airport on Monday with 170 cats and dogs evacuated from his Kabul-based animal shelter, but no local staff.

He said on Twitter he had "mixed emotions" and "true deep feeling of sadness for Afghans today."

Farthing, who set up the Nowzad shelter after serving in Afghanistan in the mid-2000s, has been campaigning since the fall of Kabul to get his rescued animals, his workers and their families out of Afghanistan.

As a British citizen, he was offered safe passage out of the country as soon as evacuations started. Visas were also granted to his 24 workers and their families – but Farthing refused to leave without the animals.

On Twitter, Farthing initially accused Defence Minister Ben Wallace of blocking the efforts to get his animals and staff out of the country.

His Operation Ark's campaign got the attention of high-level officials and political figures including UK Prime Minister Boris Johnson and divided opinion in the press and on social media, with some questioning the effort going into saving pets over people.

Farthing and his supporters – among them celebrities including British comedian Ricky Gervais – say Operation Ark did not take any resource away from the evacuation of people in Afghanistan.

But Wallace said the government offered assistance to Farthing and added that military staff received abuse online because of Operation Ark's campaign and officials were distracted by the main evacuation operations, where people were prioritized over animals.

Others agreed. "What would you say if I sent an ambulance to save my dog rather than to save your mother?" Conservative lawmaker Tom Tugendhat said on Saturday, speaking to radio station LBC.

"We've just used a lot of troops to bring in 200 dogs. Meanwhile, my interpreter's family is likely to be killed," he added.

The staff of Nowzad were unable to board a plane to the UK as they didn't make it to the front of the line at the airport, Nowzad reported, saying they were turned away as U.S. President Joe Biden changed paperwork rules two hours before their arrival.

Farthing said he was told there was no refugee to be found to fill the empty seat on his plane.

The animals traveled in a cargo hold and made it to London in good health. They've now been assigned to quarantine kennels as they're tested for potential diseases.

All Western troops are expected to leave the country by Tuesday, with the UK having flown its last military out of the country on Saturday. 

The UK's final plane carrying civilians also took off on the same day as the country wraps up its evacuation operations before the August 31 deadline. 

Since August 14, the UK has evacuated more than 15,000 people from Afghanistan. But Wallace estimates that up to 1,100 Afghans eligible for relocation under the UK's scheme "didn't make it."

General Nick Carter, head of the UK armed forces, who was involved in the evacuations, told the BBC on Saturday that eligible Afghans who had not been evacuated until now will still be welcome in Britain if they managed to leave after the deadline, through third countries or other ways.