The Rwandan army has accused the neighbouring Democratic Republic of Congo of "deliberately" firing a rocket over the border.
Rwanda's military spokesman Joseph Nzabamwita said firing the rocket, which landed near the border on Thursday afternoon, was a "provocative act".
"DRC forces today deliberately fired a rocket across the border onto Rwandan territory," an army statement said.
The incident came as fighting resumed outside the eastern DRC flashpoint city of Goma between the DRC armed forces and M23, a Congolese rebel group Rwanda is accused of backing.
Neighbours said at least five civilians were hit by mortar fire in Goma during a second day of heavy fighting.
The violence marked the first reports of civilians being wounded inside the city since late May, and prompted the UN peacekeeping mission to issue a statement saying it would take the "necessary steps to protect civilians".
I have ordered MONUSCO to react and take necessary steps to protect civilians and prevent any advance by the M23.
A witness, Dieudonne Kwibuka, told The Associated Press that at least three people were wounded by mortar rounds that fell near an Anglican church near the centre of the town.
Residents at both locations said they believed the mortar rounds had been fired by Rwandans.
"We could see and hear the direction they were coming from," said Kwibuka.
Rwanda has consistently denied allegations of involvement in the Congolese conflict or of support for the M23 movement, which is led by veterans of previous rebellions that were backed by Rwanda.
The Rwandan border is about 5km east of where the mortar rounds landed on Thursday, whereas the fighting between the army and M23 was about 13km to the north.
A statement issued by the UN peacekeeping mission in Congo, MONUSCO, said "populated areas and UN troop positions have been directly targeted and hit by indiscriminate mortar fire" in the Goma area since Wednesday.
"I have ordered MONUSCO to react and take necessary steps to protect civilians and prevent any advance by the M23," the new head of the mission, Martin Kobler, said in the statement.
Both sides have blamed each other for the resumption of hostilities on Wednesday after a three-week lull.