Part A Inclusion Criteria

* Participants with cytologically/histologically confirmed MDS according to the 2016 World Health Organization (WHO) classification with the following:

* Measurable disease per WHO MDS with excess blasts criteria as defined either:

* 5%-9% blasts in the bone marrow or 2%-4% blasts in the peripheral blood or
* 10%-19% blasts in the bone marrow or 5%-19% blasts in the peripheral blood
* MDS that is relapsed or refractory and must not have other therapeutic options known to provide clinical benefit in MDS available.
* Treatment failure after prior hypomethylating agent (HMA) therapy for MDS, defined as one of the following:

* Progression (per 2006 International Working Group \[IWG\] criteria) at any time after initiation of HMA therapy.
* Lack of response (failure to achieve complete remission \[CR\], partial response \[PR\], or hematologic improvement \[HI\] per 2006 IWG criteria) after at least 6 cycles of azacitidine (or equivalent HMA) or 4 cycles of decitabine (or equivalent HMA).
* Relapse after achievement of CR, PR, or HI (per 2006 IWG criteria).
* Intolerance of HMA (Grade 3 or higher non-hematologic toxicity leading to treatment discontinuation).
* Participants with isolated 5q-/5q- syndrome must have progressed, failed, relapsed, or not tolerated lenalidomide in addition to HMA.
* Must be off HMA therapy ≥ 2 weeks and must be off any other treatments for MDS for ≥ 4 weeks prior to first dose of SEA-CD70; growth factors and transfusions are allowed before and during the study as clinically indicated
* Eastern Cooperative Oncology Group (ECOG) performance status of 0-1

Part B Inclusion Criteria

* Participants with cytologically/histologically confirmed MDS according to the WHO classification with the following:

* Measurable disease per WHO MDS with excess blasts (MDS-EB) criteria as defined either:

* 5%-9% blasts in the bone marrow or 2%-4% blasts in the peripheral blood, or
* 10%-19% blasts in the bone marrow or 5%-19% blasts in the peripheral blood
* MDS that is relapsed or refractory and must not have other therapeutic options known to provide clinical benefit in MDS available.
* Treatment failure after prior HMA therapy for MDS defined as one of the following:

* Progression (per 2006 IWG criteria) at any time after initiation of HMA therapy.
* Lack of response (failure to achieve CR, PR, or HI per 2006 IWG criteria) after at least 6 cycles of azacitidine or 4 cycles of decitabine.
* Relapse after achievement of CR, PR, or HI (per 2006 IWG criteria).
* Intolerance of HMA (Grade 3 or higher non-hematologic toxicity leading to treatment discontinuation).
* Participants with isolated 5q-/5q- syndrome must have progressed, failed, relapsed, or not tolerated lenalidomide in addition to HMA.
* Must be off HMA therapy ≥ 2 weeks and must be off any other systemic treatments for MDS for ≥ 4 weeks prior to first dose of SEA-CD70; growth factors and transfusions are allowed before and during the study as clinically indicated.
* ECOG Performance Status of 0-2

Part C Inclusion Criteria

* Participants with relapsed or refractory AML according to International Consensus Classification (ICC) 2022 (except for acute promyelocytic leukemia \[APL\]):

* Who have received either 2 or 3 previous regimens to treat active disease. Post-remission treatments, intrathecal chemotherapy, and radiotherapy are not considered previous regimens.
* Who have received 1 previous regimen to treat active disease and have at least one of the following:

* Age \> 60 and ≤75 years.
* Primary resistant AML (defined as failure to achieve CR after 1-2 courses of induction therapy)
* First CR duration \<6 months
* Adverse-risk per European Leukemia Network genetic risk stratification
* Secondary AML (prior history of MDS or therapy-related)
* Age 18-75 years
* ECOG performance status of 0-2

Parts D and F Inclusion Criteria

* Participants with diagnosis of MDS or MDS/AML according to ICC 2022 criteria
* Disease which has relapsed, failed to respond after minimum of 6 cycles, or progressed following an HMA in the immediately preceding line of therapy.
* Eligible for continued therapy with azacitidine
* Must be off any other systemic treatment for AML/MDS. Must be off HMA therapy ≥ (greater than or equal to) 2 weeks and any other systemic treatments for MDS for ≥ (greater than or equal to) 4 weeks prior to first dose of SEA-CD70
* ECOG Performance Status 0-2

Parts D and E Inclusion Criteria

* Participants with diagnosis of MDS or MDS/AML according to ICC 2022 criteria, previously untreated.

* Participants with MDS/AML should not have AML-defining cytogenetics.
* Participants with higher-risk (Moderate High, High, or Very High) per Molecular International Prognostic Scoring System (IPSS-M) MDS and MDS/AML
* ECOG Performance Status 0-2

Exclusion Criteria (All Parts)

* History of another malignancy within 3 years before the first dose of study drug or any evidence of residual disease from a previously diagnosed malignancy. Exceptions are malignancies with a negligible risk of metastasis or death.
* Previous exposure to CD70-targeted agents
* Prior allogeneic hematopoietic stem cell transplant, for any condition
* Central nervous system leukemia based on imaging or documented positive cytology in cerebral spinal fluid
* History of clinically significant sickle cell anemia, autoimmune hemolytic anemia, or idiopathic thrombocytopenic purpura
* Parts D and F only: Prior oral HMA or oral HMA-combinations