FIFA has apologized after its website failed to cope with demand for World Cup tickets.

Many fans spent hours waiting to get into the site after sales reopened Tuesday morning, while others complained they had paid for tickets but didn't get any confirmation the tickets were reserved.

Writing on Twitter, FIFA says "we are really sorry that problems have been persisting," adding that "we've been informed that the issues relate to the sheer volume of fans accessing the ticketing platform."

Most World Cup tickets so far have been issued by a lottery with equal chances for fans who apply at any time within a given window.

However, Tuesday marked the start of two sales phases on a first-come, first-served basis before the tournament begins June 14 in Russia.