Nissan Motor Co. has only filled about 60 of the 6,000 domestic orders for its new Leaf electric vehicle because it is taking extra care in building them, officials said Monday.

The pre-ordered cars are due to be delivered by March 31, but Nissan said there was nothing to worry about.

"Since it is our first major project involving mass production of EVs, we began by building them with caution. . . . The production lines are working fine, so we expect to fill the orders as planned," one of the officials said.

The Leaf, launched in Japan on Dec. 20, hasn't been selling as well as expected. The officials said 19 units were sold in December and about 40 more between Jan. 1 and 14.

Sales have been slow partly because of the time it takes for consumers to apply for and receive the government subsidy for EV purchases.