It appears that since July 2014, there has been an international armed conflict between Ukraine and Russia, in parallel to the ongoing non-international armed conflict in Ukraine.
There is an international armed conflict between Ukraine and Russia since July 2014 in parallel to the ongoing non-international armed conflict in eastern Ukraine between Ukrainian pro-Russian rebels and Ukrainian government forces.
In addition, Russia is also occupying Crimea.
For an international armed conflict to exist, there must have been a resort to armed force involving at least two states. The threshold for an international armed conflict is very low and does not require a certain intensity or duration. The existence of an international armed conflict is to be determined by the facts, not the subjective intent of the belligerents. For further information, see the Classification section.
From the outset, Ukraine has consistently claimed that Russian soldiers were participating in the fighting in eastern Ukraine. However, Russia denies these allegations, claiming that the Russians fighting in Ukraine are volunteers, including discharged members of the Russian army. ‘Ukraine Crisis: Thousands of Russian Fighting in East’, BBC, 28 August 2014, M. Urban, ‘How Many Russians Are Fighting in Ukraine?’, BBC, 10 March 2015. Ukraine has repeatedly captured Russian servicemen in eastern Ukraine. 'Captured Russian Troops "in Ukraine by Accident", BBC, 26 August 2014; A. Luhn, 'Russian Soldiers Captured in Ukraine to Face Trial on Terrorism', The Guardian, 18 May 2015. Locals have also repeatedly claimed that Russian soldiers were fighting alongside rebels. S. Walker, A. Luhn, 'Tension High in Ukrainian Border Towns Menaced by Russian Forces', The Guardian, 30 August 2014. In July 2014, Ukraine accused Russia of having shot down a Ukrainian military plane. 'Ukraine Conflict: Russia Accused of Shooting Down Jet, BBC, 17 July 2014. Both have accused each other of cross-border shelling. 'Ukraine and Russia Claim Cross Border Fire', Al-Jazeera, 25 July 2014. Based on the analysis of satellite images and social media, there is mounting evidence of crossborder artillery shelling since July 14. Bellingcat, Origin of Artillery Attacks on Ukrainian Military Positions in Eastern Ukraine between 14 July 2014 and 8 August 2014, 15 January 2015; S. Case, Putin's Undeclared War: Summer 2014 - Russian Artillery Strikes Against Ukraine, Bellingcat, 21 December 2016; S. Frizell, 'U.S: Satellite Imagery Shows Russian Shelling Easter Ukraine, Time, 27 July 2014. Finally, there are many reports of Russian soldiers having died in Ukraine. O. Boldyrev, 'Ukraine Conflict: Russian Families Look For Soldier Sons', BBC, 28 August 2014; A. Luhn, 'They were Never There: Russia's Defence for Families of Troops Killed in Ukraine', The Guardian, 19 January 2015. All these factors combined point towards direct Russian involvement in eastern Ukraine in support of the rebels since July 2014. Hence, in addition to the non-international armed conflicts between the rebels and the government, there appears to be a parallel international armed conflict between Ukraine and Russia.
Views of parties to the conflict and other actors
The Office of the Prosecutor of the International Criminal Court concluded in its November 2016 on Preliminary Examination Activities that the available information 'would suggest the existence of an international armed conflict in the context of armed hostilities in eastern Ukraine from 14 July 2014 at the latest, in parallel to the non-international armed conflict.' Office of the Prosecutor, Report on Preliminary Examination Activities, November 2016, §169. For a contrary view, albeit reached at an earlier point in time, see R. Heinsch, ‘Conflict Classification in Ukraine’, International Law Studies 91 (2015), U.S. Naval College of War, p 354.
Both Russia and Ukraine are parties to the four 1949 Geneva Conventions and the 1977 Additional Protocol I. In addition, they are bound by customary international humanitarian law applicable to international armed conflicts. Customary international law consists of unwritten rules that come from a general practice accepted as law. Based on an extensive study, the International Committee of the Red Cross maintains a database on customary international humanitarian law. In addition to international humanitarian law, international human rights law continues to apply during times of armed conflict.