"We expect the arrival of Sergei Lavrov at about 5 p.m. Moscow time (14:00 GMT)," the source said.
The source added that more foreign ministers were coming to the Belarusian capital and suggested they could have planned a meeting of their own on the sidelines of the four-way summit, due to take place in Minsk later on Wednesday.
The leaders will discuss a way out of the armed conflict in eastern Ukraine. They are expected to build on a September 2014 ceasefire deal that was agreed on by the Contact Group of representatives from Ukraine, Russia, eastern Ukrainian independence supporters and the Organization for Security and Co-operation in Europe (OSCE).
It is hoped are that the four leaders will force the warring parties to adhere to the ceasefire agreement, in a bid to end months-long bloodshed that has taken more than 5,400 lives since the conflict started last April.
