In an interview with the BBC, Cameron said the measures taken against Russia, which is accused of supporting separatist insurgents in eastern Ukraine with weapons and uniformed Russian troops, were beginning to show results.
Russia needs to understand that if such actions continue, pressure from the West will increase, Cameron said, adding that sanctions were the right way to tell the Russians that "what they were doing was unacceptable".
US President Barack Obama also took part in talk
The British prime minister noted that the Russian economy had been growing and was now contracting, while the country's banks, stock market and currency were also feeling the pain from the sanctions imposed by the United States and other Western countries.
Cameron spoke with the BBC as he prepared to host Thursday's NATO summit in Newport, Wales.
Ukrainian President Petro Poroshenko and the leaders of several former Soviet republics, including Estonia, Latvia and Lithuania, attended the summit.
US President Barack Obama also took part in talks centered on the situation in Ukraine, the crisis in Iraq and the scheduled departure of the NATO-led international force from Afghanistan by the end of the year.