"What has been achieved so far indicates that if the war was to last longer, the scope of sanctions would increase," Rau said after holding talks with his German and French counterparts in central Poland.
"There is agreement between us on this," he added.
The Polish, German and French foreign ministers got together for a Weimar Triangle meeting in the central Polish city of Łódź on Tuesday to discuss Russia's invasion of Ukraine.
Rau hosted the meeting in his role as Poland's top diplomat as well as chairman of the Organization for Security and Cooperation in Europe (OSCE).
400,000 refugees sheltered by Poland so far: FM
Rau told reporters after the meeting that Poland has taken in around 400,000 refugees since the start of the conflict in Ukraine.
He added that the number was likely to grow to "reach 1 million" as more people arrived into the country.
'War crime under international law'
Rau said in a tweet on Tuesday that the "shelling of Ukrainian cities and targeting civilians" by Russian forces "must be condemned by all members of the international community."
"It’s a war crime under international law," he added.
He called on Russia "to respect Article 25 of the Hague Regulations of 1907 and all provisions of the 4th Geneva Convention."
'Time to welcome Ukraine into EU community'
In a tweet on Monday, Rau said that "it's high time to welcome Ukraine into the EU community" as "the Ukrainians heroically defend themselves against Russian aggression."
He vowed that Poland "will provide all necessary assistance to Ukraine's accession process."
On Sunday, Rau called for an immediate ceasefire between Russia and Ukraine to let evacuate civilians and deliver humanitarian aid to the city of Kyiv.
The Weimar Triangle group was set up by Poland, Germany and France in the early 1990s to help overcome divisions in Europe.
Earlier this month, Poland's President Andrzej Duda urged a united stance by the Western community in support of Ukraine during talks with France's Emmanuel Macron and Germany’s Olaf Scholz in Berlin.
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Source: PAP, Reuters