Senegal's President Macky Sall opened the meeting attended by 12 of the 15-nation Economic Community of West African States (Ecowas) leaders, aimed at "synchronising the regional response" to the political crises.
Ecowas chairperson, Ivory Coast President Alassane Ouattara condemned "the refusal of the Mali and Guinea-Bissau juntas to accept Ecowas decisions."
"The defiance of the two military juntas is leading us to take further steps to implement our organisation's decisions," he added.
Six Guinea-Bissau coup leaders were slapped with an assets freeze and travel ban by the European Union as the meeting opened, just days after Ecowas imposed targeted sanctions against the junta.
The Dakar summit comes eight days after the regional leaders met in Abidjan, scrambling to find a solution to the new setbacks in a historically troubled region.
Senegal's President Sall raised particular concern about Mali, where plans to transition to a civilian government have been hit by military clashes.
"Recent developments in Mali are a source of grave concern. Africa and the world are watching us," Sall said in his opening speech.
At the talks was Mali's interim president Dioncounda Traore, appointed to lead the country back to democracy after a March 22 coup ousting leader Amadou Toumani Toure.
22 dead
Toure was overthrown by a military junta headed by Captain Amadou Sanogo, who quickly accepted an Ecowas-negotiated deal to hand back power to civilians.
However the soldiers never really stepped back and have continued to make arrests, targeting former allies of Toure.
On Monday and Tuesday, elite "Red Beret" paratroopers who had remained loyal to Toure attempted a counter-coup and tried to seize the airport, national broadcaster and a military barracks that has become the headquarters of the ex-junta.
Hospital officials told AFP that at least 22 people were killed in fighting between the ex-junta and presidential guard.
However Sanogo continues to assert his authority. On Saturday, he rejected an Ecowas demand for elections in Mali within 12 months.
He also has rejected a plan to send foreign troops into northern Mali, captured by a loose coalition of Tuareg and Islamist rebels following the March 22 coup.
In Guinea-Bissau, plans for a return to constitutional rule are clouded in ambiguity as the junta prevaricates and pressure mounts from abroad.
Army chief Antonio Indjai led a coup on April 12, aborting an election process in the country which has a long history of military overthrows and chronic instability.
The junta initially accepted a 12-month transition programme brokered by the region, however three days later talks in Banjul collapsed and Ecowas slapped the coup leaders with sanctions, saying Indjai was not willing to negotiate.
Citing in a statement the "seriousness of the current situation in Guinea-Bissau" the EU approved targeted restrictive measures against six of the coup leaders.