Union dockworkers at more than 30 ports in the United States have gone on strike, raising concerns about extensive impact on the economy.

About 45,000 union workers at ports mostly in the eastern and southern US went on strike on Tuesday because the union failed to reach a deal with the employers' organization by a September 30 deadline.

They had been negotiating the percentage of wage hikes and the use of automation technology. The employer side reportedly offered at the last minute nearly 50 percent pay raises over six years.

The ports on strike are located in several states, including New York and New Jersey in the east, and Texas in the south.

Reuters news agency says those ports handle about half of the US cargo from ships, including food and medicines.

According to one estimate, the shutdown of the ports could cost the economy up to 4.5 billion dollars per day. A range of businesses, including retailers handling perishable food products, could be heavily impacted.