The Coast Guard (search) on Wednesday began imposing $10,000 fines against shippers and ports that missed a year-end deadline to submit terrorism protection plans.

Congress last year ordered the shipping industry to tighten security amid such fears as a hijacked oil tanker rigged with explosives or a radioactive dirty bomb smuggled ashore in a shipping container. An attack on a port could kill thousands, cause massive property damage and cost the U.S. economy tens of billions of dollars.

Port facilities and ship owners had until Dec. 31 to turn in vulnerability assessment reports and security plans, but only about 42 percent had complied by year's end. The Coast Guard said that number has risen to 90 percent.

"We have made tremendous progress protecting the ports, and we need everyone to continue that progress by meeting those requirements," Rear Adm. Larry Hereth, the Coast Guard's director of port security, said in a statement.

The plans are a prerequisite for a July 1 deadline requiring ships, ports, ferry terminals and fuel-chemical tank farms to implement tighter security measures. The Coast Guard said it has started to impose fines of $10,000. Those that don't comply face more fines of up to $25,000 and could be shut down after July 1 if approved security plans aren't in place.