An offshore union has warned more North Sea strikes could take place by summer.

Unite members are preparing to vote on a fresh pay offer from the Offshore Contractors Association (OCA) after turning down an earlier deal.

The union has threatened that a vote to reject the deal could lead to "sustained strike action".

Hundreds of oil workers took part in the first North Sea strikes in a generation last summer.

In a letter to members seen by STV News, Unite wrote: "Should the OCA pay offer be rejected then it's more likely that we will be looking at sustained periods of strike action over the coming months and possibly throughout the summer should the return from an industrial action ballot give union members that legal mandate to strike."

Unite wants all OCA employees to receive a pay increase under any new agreement.

However, the OCA had said only workers paid below the minimum rate would automatically benefit and it would be down to individual companies to negotiate with staff.

The OCA, which is the leading body representing offshore contractors in the North Sea, welcomed the news that Unite members would be balloted on its proposal.

Chief executive Paul Atkinson said: "This consultative ballot follows some positive negotiations that have focused on reaching an agreement that will help protect and sustain North Sea employment now and in the future."

The OCA's offer guarantees that basic and sick pay will increase by a minimum of 2% in each of the next two years.