Six regional airports remain closed until the start of Friday due to a strike by air traffic controllers locked in a row over pay.

The airports at Inverness, Dundee, Benbecula and Stornoway in the Outer Hebrides, Sumburgh in Shetland and Kirkwall in Orkney have been shut on Thursday.

The Prospect union is seeking a 10% pay increase for staff and said it has been left with no choice but to take action after running out of patience with operator Highlands and Islands Airports Limited (Hial) and the Scottish Government.

Hial, a public corporation wholly owned by Scottish ministers – insists it is bound by Government pay policy and cannot negotiate a separate settlement.

It said it is “extremely disappointed” by the escalation to strike action, which it said has affected more than 6,000 customers.

On Thursday, the union praised what it described as a strong response to the strike from members.

Prospect negotiator David Avery said: “We did not want to have to take this action today but our members have lost patience with the lack of engagement from Hial and Scottish ministers.

“We are pleased with the strong response from our members which was exactly what we expected given the overwhelming response to the strike ballot.

“The emphasis is now on the employer and on ministers to come back with a serious offer so that we can move forward.”

It is understood a regular meeting with Hial is scheduled for Monday, when the issue of pay is expected to be brought up.

Hial has said it believes its air traffic controllers are well paid and has called for the union to moderate its claim.

Managing director Inglis Lyon said: “I’m extremely disappointed that this dispute has escalated to the strike action today.

“It is a huge regret and I want to apologise for the inconvenience that this has caused our customers.

“Over 6,000 customers across the Highlands and Islands have been hugely inconvenienced.

“There is also the knock-on economic consequences, including the impact to the small businesses who operate at our airports.”

He added: “Prospect, a London-based union, feel the Scottish public sector pay policy should not apply to Hial air traffic controllers and their claim remains 10% but the policy exists and Hial are bound by it.

“I would urge Prospect to moderate their claim and sit with us to have open, honest and constructive discussions to allow us to jointly resolve this dispute.”

The Scottish Government said it is encouraging both sides to return to discussions and to work towards resolving the dispute.

A spokesman said: “We have been clear with both Hial and Prospect that any settlement must be in line with Scottish public sector pay policy.

“Hial has implemented a pay rise for all staff, which is a significant improvement on previous years, as well as significantly increasing their contribution to their pension scheme in order to maintain this benefit for employees.

“Staff also continue to receive generous annual leave, sick pay and other allowances.

“In the face of the UK Government’s continued budget cuts, the Scottish Government delivered a distinctive and progressive pay policy for 2018-19 – one which is fair, supports those on lower incomes and protects public-sector jobs and services while delivering value for money for the people of Scotland.”