Peacekeeping mission condemns ‘appalling act’ as it calls for immediate end to violence against civilians in the east.
The armed forces say the rebel group also violated a five-day-old ceasefire in the east of the country.
Doses will be used first for health workers and people living in hardest-hit areas, especially DRC, Ghana and Nigeria.
The Vatican had previously announced that scheduled trips to both countries in July were postponed.
The election, expected to cost $600m, comes amid ongoing violence by M23 rebels in the restive eastern region.
The armed group was warned that if they did not cease fire after the deadline, force would be used to push them out.
Congolese government has rejected any direct talks with M23, while the armed group says it has no plans to withdraw.
Regional African leaders, including from Rwanda and DRC, agree on the cessation of hostilities from Friday.
The M23 armed group is marching on Goma, the regional capital in eastern DRC.
Uganda will be the third country to deploy troops after contingents from Kenya and Burundi have arrived in eastern DRC.
Rwanda’s defence ministry said an unidentified man opened fire in Rubavu district before he was shot dead.
Kenya’s former president and Rwanda’s leader agree on need for M23 rebels to cease fire and withdraw from eastern DRC.
The time has come for the restitution of myriad African artefacts and works of art looted in the colonial era.
Fighting between the M23 and the DRC army has led to a looming humanitarian crisis as thousands have been displaced.
