WASHINGTON, April 23 (Reuters) - The average rate on the popular U.S. 30-year fixed-rate mortgage fell this week, though further declines are likely to be limited by uncertainty surrounding a fragile ceasefire between Washington and Iran.

The 30-year fixed mortgage rate averaged 6.23%, down from 6.30% last week, mortgage finance agency Freddie Mac said on Thursday. It averaged 5.98% just before the U.S. and Israel attacked Iran at the end of February as Freddie Mac and Fannie Mae expanded purchases of mortgage-backed securities. 

The rate shot up to an average of 6.46% at the beginning of April. Mortgage rates track U.S. Treasuries, which have largely been range-bound amid lingering uncertainty over the truce. 

Trump on Tuesday indefinitely extended the ceasefire with Iran, though a U.S. Navy blockade of Iranian ports remained in effect. On Thursday, Trump said he had ordered the U.S. Navy "to shoot and kill any boat" that is laying mines in the Strait of Hormuz, lifting global oil prices.