ROME, May 29 (Reuters) - The busiest week of migrant crossings from Libya towards Italy this year may have led to the deaths of at least 700 migrants at sea this past week, Medecins San Frontieres and the UN Refugee agency said on Sunday.

About 14,000 have been rescued since Monday amid calm seas, and there have been at least three confirmed instances of boats sinking. But the number of dead can only be estimated based on survivor testimony, which is still being collected.

"We will never know exact numbers," Medecins San Frontieres said in a Tweet after estimating that 900 had died during the week. The United Nations High Commissioner for Refugees (UNHCR) said more than 700 had drowned.

(Reporting by Steve Scherer; Additional reporting by Reuters TV in Reggio Calabria; Editing by Richard Balmforth) ((; +380442449150; Reuters Messaging: